Are you playing civ 3 Complete or just Civ 3? I'm asking so I can figure out if you're playing a scenario or just starting a game.
Could be a couple of things. check what victory conditions are set when you start the game is the big one. And what year does it say it is when your game ends. Also, make sure you didn't tweak the game settings, especially the turns # (should be 540 by default).
There's a couple of us here who still play Civ 3 a lot so ask away