Can you use 3 12 volt batteries instead of 6 6volt batteries on a golf cart?
That depends on how the batteries were wired, for what voltage.
If they were to provide a 6 volt source, then yes, but not for very
long as the 12 volts would burn the motors out very quickly. If
they were wired to provide for 18 volts then yes, but it won't run
very fast because of the low voltage.