PS3 has to be up there because it has well over 10 excellent exclusives, games that you'll never be able to play anywhere else.
The Wii doesn't have enough AAA games to compete with the PS3's list, unfortunately. All you have is Metroid, Mario, Zelda, Wii Fit, Wii Sports, Brawl, Mario Kart, and then a black hole. It is tempting to include it simply because of the motion control, but in terms of overall quality and quantity, it lags too far behind to be a viable option.
The 360 is a non-factor if PC is included. You have far less exclusives than the PS3, and a lot of games that are better on PC. The 360 exclusive list of awesome games essentially drops down to Halo 3, ODST, Gears of War 2, Forza 2 & 3, and that's it. If you pick 360 in your list of three when PC is an option, your logic is fanboy logic.
The PC is an obvious choice if you're picking three. If you have a good enough system, you'll be able to play a plethora of games in higher detail than their console counterparts. Bioshock, Modern Warfare, Left 4 Dead, Red Alert 3, the list goes on and on. Additionally, you're open to dozens of excellent indie games and mods. PC is an obvious choice.
For handhelds, it really comes down to the PSP and the DS, and sadly the choice is obvious. The PSP is a great system with some excellent games worth buying, but the DS has so many great games spanning so many genres that I lost track long ago. It's the premiere RPG system, 2-d platformer system, and is still filled with games to fill other genres, some being new (Trauma Center, Ace Attorney, Elite Beat Agents, Meteos, Training games, Picross, top down shooters like Nanostray, etc).
So, pretty much, PC + PS3 + DS.
Currently playing: Uncharted: Drake's Fortune, NBA2k11, Metal Gear Solid, Picross 3d