Which three did you press?
If you chose C M XC IX, you've solved the puzzle. The door unseals (magically or otherwise), and slides down into the floor. Whatever lies beyond it is up to your game system, though probably ought to be a fairly decent treasure -- and perhaps its guardian.
If you chose anything else, your PC(s) won't be able to solve the puzzle either. They will have to consult a specialist of some sort, be it sage, wizard, priest, or historian.