The factory cap would be metal, usually with a 3/4" square head on it, though I have seen a few with a hex head. The plastic one is an aftermarket replacement. It should have 2 tabs on it, like a wingnut, to allow for getting ahold of it for install/removal. You may have to resort to the punch and hammer method to get it loose. Replacement caps (plastic) are available from the parts stores, though you will have to order it.
DOT 3 is the correct fluid. I would suggest filling the system, bleed it, then check for leaks. If the fluid is indeed low, you most likely have a leak somewhere, possibly the master or wheel cylinders if you do not see anything obvious.