There have been some films which have been presented in trilogy - the top 10 are given below --

(10) Ice Age  - an animated film (2002)
Ice Age
Ice Age : the Meltdown (2006)
Ice Age : Dawn of the Dinosaurs (2009)

(9) Toy Story - an animated film (1995) - the First Animated Full Length Movie
Toy Story
Toy Story 2 (1999)
Toy Story 3 in 3D (2010)
Toy Story and Toy Story 2 in 3D (2009)

(8) The Matrix (1999)
1 - The Matrix
2 - The Matrix - Reloaded (2003)
3 - The Matrix Revolutions (2003)

(7) X-Men Origins: Wolverine (Their Prequel) (2000)
X-men Origins
X2: X-Men United (2003)
X-Men: the last stand (2006)

(6) Jurassic Park (1993)
1 - Jurassic park
2 - The Lost World : The Jurassic Park: (1997)
3 - The Jurassic Park III (2001)

(5) The Lord of the Rings :
I - The Fellowship of the Ring (2001
II - The Two Towers (2002)
III - The Return of the King (2003)

(4) Pirates of the Caribbean :
1 - The Curse of the Black Pearl (2003)
2 - Dead Man's Chest (2006)
3 - At World's End (2007)

(3) Star Wars - Episodes IV-VI
IV - A New Hope (1977)
V - The Empire Strikes Back (1980)
VI - Return of the Jedi (1983)

(2) Spiderman (2002)
Spiderman
Spiderman 2 (2004)
Spiderman 3 (2007)

(1) Star Wars - Episodes I-III (released in IMAX also)
I - The Phantom Menace (1999)
II - Revenge of the Sith (2005)
III - Attack of the Clones (2002)

Some More Trilogies or in Four

Jaws I, Jaws II, Jaws III

Lethal Weapon; [Mel Gibson and Danny Glover]
Lethal Weapon-2; [Mel Gibson and Danny Glover]
Lethal Weapon-3; (1992) - [Mel Gibson and Danny Glover]
Lethal Weapon-4; [Mel Gibson and Danny Glover]

MIB: Men in Black [Will Smith]
MIB: Men in Black 2 [Will Smith]
MIB: Men in Black 3, in 3D and in IMAX 3D (2012) [Will Smith, Tommy Lee Jones]

Mission Impossible I,  II,  III [Tom Cruise]; 
Mission Impossible IV Ghost Protocol - has also been released in (2011 Dec) -

Terminator; (1984) -  [Arnold Schwarzzenegar]
Terminator 2 (1992)
Terminator 3
Terminator 4 -

Transformers
Transformers 2
Transformers 3

Rocky    -  [Sylvester Stallone]
Rocky II
Rocky III
Rocky IV (1985)