Well, now that E3 has come and gone and we know what's coming. We can expect to see all three systems to slow down in the coming months, since summer is a very slow time when it comes to sales.

The PS4 will, I expect, will continue to trail behind 2017 for the most part, while the Xbox One continues to pull further ahead. Then, get a huge jump in sales in September and October from FIFA & Spider-Man, and CoD, Battlefield, and RDR2, respectively. That jump in October will go for the Xbox One as well, maybe even more so with the addition of Forza Horizon 4. However, I expect the Xbox One to fall behind significantly in November and December since that was the Xbox One X launch period last year and they don't have anything to match it this year. Unless they do very cheap deals that like a $299 Xbox One X, highly, HIGHLY unlikely though. I think it's going to be a pretty close call for Microsoft this year. The majority of people thought they were the winners of E3 this year, which I also agree with. Let's see if that boosts momentum and sales in some capacity. It's a matter of will 2018's lead be big enough come that first week of November once shit hits the fan.

I also expect the PS4 to be behind as well in the holiday months. They may have some deals in store like they did last year, but I don't think they're going to have as big of an effect. I feel as the the system is beginning to lose some steam and reach that saturation point. Sony themselves are expecting to ship 18 million for the fiscal year, down from 20 million last fiscal year: https://www.theverge.com/2017/4/28/15466956/playstation-4-sales-sony-forecast, coming out of E3, I can see why. With no plans for an official price cut in sight, '2018 PS4' is fighting a losing battle here.

Last but not least, the Nintendo Switch. I expect to be continuously up YoY between now and September. Remember how last year during the summer time, the Switch was going through severe stock issues that slowed sales considerably as Nintendo struggled to keep up. Since that obviously won't be the case this year and there's a healthy supply of games and content this summer to keep fans busy and attract buyers (Mario Tennis Aces, Crash N Sane Trilogy, Project Octopath Traveler, Okami HD, etc.) The Switch shouldn't have any trouble being consistently up YoY these next few months from June to September, with the exception of the last 2 weeks of July (Splatoon 2 launch period last year). It will most likely be down in October from last year. As great as Super Mario Party looks, that's no match for Super Mario Odyssey. And this will spill over into the first week of November and I think 2018 Switch still be trailing those first two weeks. But, man oh man, once that 2nd week hits and Pokemon arrives, followed shortly by Black Friday and then finally, Smash Bros. LOOK OUT!

Pretty much exactly what I'm expecting, too. However, the way the sales curve is going, I think that the PS4 will permanently lower the price of the 1TB slim to 249$/€, otherwise it will be extremely hard to even hit those 18M. At current pace the PS4 will be 1M behind last year and 2M by September - and those numbers are inflated for a fiscal year due to the MHW boost early this year.

On the other hand the E3 presentation of Microsoft combined with all the pricedrops throughout the year seem to push the console high enough that it could still be up YoY. I think the sales in the holiday season will be up WoW, the question being will it be enough to counter the X launch? I pretty much think so.

June 2nd (Week 22) Numbers have been added.
All three consoles saw slight increases over the week before with the PS4 and Xbox One cracking 6 million and 2 million on the year, respectively.
However, 2018 PS4 has fall behind to half a million now and is only going to get worse from here. Meanwhile, 2018 Xbox One has increased its lead to over 300k and will only get better from here. 2018 Switch also increased its yearly lead and given how 5 of the next 6 weeks for 2017 will be down considerably due to stock issues around that time, so the gap will only increase even further for 2018.

Switch is doing better than last year what do you mean lol ? Also PS4 is still down with a better 1st half in 2018 compare to 2017... Let's see if it will drown during summer or actually keep a good pace. 

Switch on the other hand had a drought during first half and the games are coming for the end of June and July as far as we know. It will likely pick up a bit of speed again and will blow everything during the holidays. Switch outsold considerably the PS4 in their 1st year and will likely do the same with the second. 

6m to 2m.... PS4 is doing x3 of the XB1 world wide.
Switch is haveing a pretty decent year too.

PS4: I would say that PS4 has already peak last year but their wild card is PS4 owners converting to PS4 Pro added to casual gamer picking much cheaper PS4 consoles.
September should be bigger month with the release of Spiderman (But does this game have limited appeal? can it sell over 8 million copies and get in the top 10 PS4 games?)

I don't think Sony will do better then 2017, but they will still easily out sell Microsoft or Nintendo worldwide. 

XB1: Microsoft will do very well when compare to their own performance: (I expect that Microsoft will rebound from last year and this could be the Console peak year with 9m+) 
1. They won E3 (IMO)
2. The Xbox one X will benefit from Sony not giving any news on the PS5 and it seem more likely like next gen of consoles won't be till 2020 or later. 
3. Cheap Xbox one S with game pass is great value proposition. 

Switch: Is on pace to already have a better year then 2017, expect a big boost in the holidays while people are still interest in picking up the new consoles and Smash coming out in December just in time to boost Xmas numbers.
Disappointing E3 for Nintendo (IMO) and we are getting a slower more Nintendo like pace to game release.  

I really hope that Nintendo drop the 3DS entirely or the system only get Switch port going forward. Focusing only on one consoles should really increase Nintendo output of games... compare to Wii and WiiU. 

They're basically doing that already. The 3DS was practically nonexistent at E3 this year with no new announcements whatsoever. The only notable releases left for it from here on out, are Captain Toad (a Wii U port that is also on Switch), Mario & Luigi: Bowser's Inside Story (A DS Remake), Luigi's Mansion 1 (GameCube remake). The only actual new games that are coming out for it are Wario Ware Gold and Etrian Odyssey X. That's it.
All it takes is a glance at E3 this year, and even last year, to clearly know where Nintendo's priorities are. The 3DS, at this point, is reduced to just cheap ports that they can scrap together when they need a notable release that can make easy money.

Yeah, at the current rate PS4 will be over one million behind last year by end of July, while the Switch will be over 1 Million in front of last year by then. By September the PS4 will already hit the 2 million mark, while the Switch will get close to a 2 million lead over last year.

That all is of course only if the tracking is correct and we're not running on false data here. 

You know games arent made instantly. There is always a drought after a launch period. Look at any gen you want, PS4/One being the worst imo taking like 3 years to start seeing good games worth transitionning first party or even third party wise.

Switch will get more third party support and more first party game starting next year. Its obvious, Reggie said it, even Panic said it.. devs were just surprised and werent sure to jump in after what happenned to the U

Once we hit the end of year, I expect the Switch to be at least 3 million ahead with the PS4 being 3 million behind. Which would put each system at at around 16 million and 17 million minimum respectively.

Damn, I might have bring back my 2015 and 2016 PS4 charts after all!