So you have the game without the expansions already installed, and have bought the expansions? Just make a second install of the original in a different folder, update that one with the expansions, and that gives you the option of playing the original or the expansions.
If you got Civilization Complete, then that should install to a different folder than your old install (if not, choose a different folder). I had Civilization Gold (PTW expansion, no Conquests) before I got Civilization Complete. The old install of Civilization Gold is in a folder called "Sid Meier's Civilization III Gold", the new install is in a folder called "Civilization III Complete", so I can go back and play the Gold install if I wanted.
Though now that I look at my files, the old install of Gold is redundant, because Civilization Complete comes with a subfolder containing the PTW Gold version anyway.