1 - Well, I did play they all on the highest diffult available
2 - Depth isn't measure by the amount of movement options. Chess, Go even Checkers have very simple and limited movement options and they still can be very depth in its playing.
You may not like their choices to make the game harder, but it still is harder than GoW. And go for TLOU on grounded difficult that you are basically scrapping for single bullets and have to try to pass almost all stages without firing and also have disabled hearing plus enhanced discovery by the enemies. So there is several different changes to the ultimate difficult.
3 - Combos in GoW are very easy to do and playing it regularly it's also easy to memorize and know which enemies to use each. As I said having limited options can make it more deep than having 100 options that if you know wich of them to use at the time make it even easier and perhaps shallow.
1. You said you played on hard. There is a very hard mode. Were you saying highest available initial setting?
2. Mechanical depth in a game is measured by the mechanics. Uncharted has shallow and straightforward mechanics without any variance. God of War games have a deeper pool of combos hence have greater depth. You can't compare it to a sport.
3. Doing each combo by itself is easy. Chaining all those combos like in the video is incredibly difficult and takes phenomenal skill. I told you, the game developers themselves were amazed it was possible.
You might not have watched the videos. You should. It not only gets my point across but is entertaining as well.
For me, they were long before it.
I'm not super invested in this debate as a whole, but I do feel the need to chime in on this. Having played both titles fairly extensively back in the day, GoW3's combat has nowhere near as much depth as DMC3. It's important to note that there's a significant difference between simply having a truckload of moves that you can look cool with and having a significant degree of depth to your combat system, and while GoW3 (and the entire first GoW trilogy) has the former, it pales in comparison to DMC3 on the latter.
Combos in GoW3 that basically infinitely stunlock most enemies are incredibly easy to perform even on Chaos, and while you can go out of your way to look impressive by switching weapons on the fly and throwing in different attacks, there's no real gameplay incentive to do so; you won't be doing more damage and the game doesn't really have any other method to encourage moveset exploration beyond that. As long as you're familiar enough with enemy attack patterns to know when to drop your current combo and get out of the way of an incoming attack, you have the knowledge you need to beat the game, even on Chaos.
DMC3, on the other hand, has a much greater degree of depth because it has, at the risk of sound pretentious, let's call them "high level techniques," that are almost essential for the player to familiarize themselves with if they want to make it through the higher difficulties (Very Hard/DMD) without having to spam healing items. If you're not, at the very least, familiar with using different styles, jump canceling, and DTEs, you're going to have an extremely difficult time getting past even the first third of the game. There are advanced strategies that the player essentially needs to learn if they want to survive higher difficulties. On top of that, the game's inclusion of a style meter, influenced by the variety of attacks a player uses, not only provides a constant incentive to use different weapons, but also factors into how much in game currency you earn at the end of a level, adding an additional incentive on top of that.
Devil May Cry 3 handles its gameplay depth better than God of War 3 because of the things you stated. Doesn't make it lots deeper. Its just that Devil May cry 3 has better AI and enemy waves and the style meter is amazing incentive to mix it up. Still God of War allows nearly the same skill ceiling just doesn't require you to be that good to beat the game.
I would say it does require lots of skills to do the Challenge of the Gods in God of War 3. In the campaign, Chaos mode requires you to be really good but not to the extent that Dante Must Die in Devil May Cry 3 does. Still the option is there for those who need that depth. Watch the videos I linked there. The developers didn't know such combos were possible and that shows the amazing depth it has.