That driver chip has darlington outputs so it will be reducing 6V down to 4 or so anyway, so 3V motors aren't going
to be massively overloaded (still its desirable not to overdrive motors, they get hotter and wear out faster).
Change those 4AA cells for 4 NiMH rechargeable AA's and that'll be about 5.2V instead of 6.0V, could help (with
the voltage and with the battery draining).