Oh, come on, just three??? Oh, I couldn't choose. High up there are definitely the Raiders March, Star Wars, and Superman. For a more varied and !WARNING!DETAILED! approach, here's some of my favorites:
Back to the Future
(Alan Silvestri): Very exciting, action packed theme that is both fun and shows the intensity and adventure of the movies.
(James Horner): A couple of main themes. The main theme, heard in the opening titles sequence for the movie, is a softer, but heroic theme that is performed wonderfully in the movie. Then there is the secondary theme, that appears more in the adventures of Cliff when he's using the rocket. A very good, and faster, theme than the main theme.
(John Williams): Though this score is extremely thematically rich, my favorite, being a softie for fun and adventurous themes and scores, is the Flying theme. It is heard in the "Prologue" track on the soundtrack or the theatrical trailer, and later performed full blast at the end of the Remembering Childhood scene. A really fun, swashbuckling theme with lots of re-hummability!
(John Williams): Both of the main themes, Journey to the Island and Welcome to Jurassic Park (as I dub them according to their primary scenes. Yeah, they're both the same track, but you know...) are terrific themes, as is generally expected with John Williams. JttI is a grand, sweeping theme, and WtJP is a slightly calmer, sweeter theme. Both are a great listens.
Yeah, I know, it's detailed and all, and I probably shouldn't have done all that, but I already took the time to type it out, I'm not erasing it!