A spot meter isn't necessary but carrying another camera around, merely to measure exposures, is a bit of a pain. You could get hold of a second hand Leica MC meter at very reasonable price - not the best meter ever but equivalent in ability to an old Weston meter. It fits tidily on top of your M3 and ties in with the shutter wheel.
I used one on my old M2 for many years, shooting b&w.
Alternatively, if you've got a bit more spare money, look for a more accurate, battery powered, MR meter.

Regards
Jerry