So many soundtracks, so little time. I'll give you more than 3 though. I'm a big fan of soundtracks.
First up would be the Passengers soundtrack.
I shouldn't have to explain this one.
The Big Chill - there's a couple of volumes, mostly older tunes from the 60's and such. A nice mish mash of the reminiscent, all fantastic songs.
The Who's "Tommy" - need I say more. The Who just rock anyway.
O Brother Where Art Though - get your roots on. May be too "hillbilly" and folky for some people's tastes and there was quite a commotion when this beat out U2 at the Grammies but this soundtrack is slick, and big props to it for getting people into the roots of rock.
Trainspotting - just get the first volume. A mix of great songs from Iggy Pop to Underworld, a great compilation of different types of music styles that go amazingly well together given the context of a soundtrack album.
The Virgin Suicides - there's actually "2" soundtracks. One is more like a score done by the delicious band called Air, and it happens to be one of my favorite soundtracks. Kind of a retro psychadelic 70's feel, dashed with electronica as well as organic instrumentation. The overall effect is very Pink Floydian and that's a compliment. The second soundtrack is more a mish mash of 70's rock and such.
Human Traffic - this is the quintissential "I'm going out to shake my thang" soundtrack that's perfect to play in your car on the way to a night out on the town. But if you hate dance music, then it might not be your thing.
Selma Songs - the soundtrack to Bjork's Dancer in the Dark. Either your like Bjork or you hate her. But if you like her and find her style innovative, this soundtrack will make your heart ache. It's haunting and beautiful and sometimes painful. It hurts so good though. But see the movie first...
Fantasia - it may seem cliche, but the music for an animated classic like this is extrordinary.
Sid and Nancy - this may be out of print, but it's worth it if you can find it.
And that's it for now. Hope I got all the bases covered.