First off, if we are talking stripped screw holes in plastic, put a few drops of quick drying CA glue in the hole(do not fill to the brim! half way or so will do), then drive in the correct screw normally. As it dries up, back off the screw a little to make sure the screw itself doesn't bond to anything, then drive in fully after a few seconds.
Very handy fix I learned.
I'm not sure if traxxas is imperial or metric, check your manual. Hex size is somewhat independant(ish) to thread size. Different styles of screw heads will have different hex sizes for a given diameter.
M3 refers to metric, 3mm diameter thread, not the head size. The x12 is indeed the length, but it includes the head for countersunk screws.
You'll need this chart.
Shamelessly stolen from the internet.