Monkey Island 1-3

1 - Xenosaga
2 - Atelier Iris
3 - Metroid Prime

Resident Evil 
Mass Effect 
Dragon Age 
Legacy of Kain 
Uncharted
Dead Space
Prince of Persia
The witcher
Hitman

Not in any particular order.

Gothic and Myst.

I think there are much more duologies, that would be really hard to pick.

-Atelier the arland trilogy
-FFXIII trilogy
-Super Mario Bros
-Silent Hill

I do think Uncharted and God of war series are awesome but havent played UC 1 and GoW 3 so. i cant put them on my list :P

Well if you have a PS3/PS4 you should play GOW III NOW.

UC 1 is skippable if you've already played UC 2 or 3.

Mother trilogy
SMB trilogy
Classic Sonic trilogy
Crash Bandicoot trilogy

the reason i didnt play GoW 3 is because my younger brother did it and i was sitting next to him when that happen. I know is not the same, but i didnt feel like playing the game afterwards.... Still, it was awesome to look at XD

Depends on the definition you use here. I could take three games out of a series that's longer than just three games or contains multiple trilogies in one and make quite the list. This discounts a Crash Bandicoot, Mario, Zelda, Metroid, Resident Evil, Professor Layton, Jedi Knight and whatever else. I could take a series where I didn't play all three games and count it anyway, but I won't, which discounts a couple more like Bioshock, Diablo, God of War or Rogue Squadron.

However, taking series that consist of only three consecutive games that I played through to the end, there isn't much.

Basically all that remains that fits the qualification of being a pure trilogy that I played to completion is; Uncharted. Which does happen to be one of my favorites indeed, but soon, it won't count as a trilogy anymore either.

1. Crash Bandicoot trilogy

2. Grand Theft Auto 3 trilogy

3. Gothic trilogy

4. Uncharted trilogy

5. Ratchet & Clank trilogy

6. God of War trilogy

7. Assassin's Creed Ezio trilogy