Very hard to come up with 3 once you really start to think about it.
I would go with (in no particular order):
Alpha Centauri - The sheer amount of fun hours I put into this game are impossible to overlook. I remember the day I installed it and was overwhelmed by the interface (I actually played Civ 2 after Alpha Centauri). Within a day I was off and building my first empire.
I just played a game of Alpha Centauri not too long ago. I bought it in 1999. 10 years of playability ain't too shabby.
Gothic - The decision between Gothic and Gothic 2 is a tough one, but ultimately Gothic gets the nod for me because Gothic 2 wouldn't have existed without it. The prison colony within the dome is the most immersive world I have ever experienced in a game.
The combat in unintuitive at first, but after you spend some time mastering it the reward is 10x as great as the button mashing in games like Morrowind.
Baldur's Gate - my last semester in college I had no money and was stuck in a crappy one room apartment with nothing but my laptop for entertainment (I didn't even have TV).
One day, bored put of my skull, I decided to use whatever cash I could scrounge up to buy a game to keep me occupied. Luckily, Baldur's gate was only $19.99 by then - otherwise I couldn't have afforded it.
The rest is history for me. I bought ToSC a few weeks later and spent the rest of the semester (2+ months) walking around in that world.
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