Unable to Power Arduino and DC Motors via 2 AAA Batteries
The problem seems to be you're trying to take out 2*700 * (5V/3V) = 2.33 A from the AAAs, which is problematic. Even more, the batteries have internal resistance (which lowers the 3V) and you have DC converter losses.

I guess a pair of good NiMHs might be capable of providing that current (I've seen 5A briefly), but then again, you're still massively out of spec of the components in your power system.