we don't know where you live, so we can't guess whether you have hills are not.
My daughter has a Nexus 8 - her first full size bike, purchased to get her anticipating shifting in our hills. I turned it into touring gears by changing the front chainring down by 2 teeth and increasing the rear drive cog up by two teeth.
It's geared to climb a 14% grade now.
If you live in relatively flat, whether you plan to use your motor a lot, you can get by with the 3.
If you need to climb steep hills, you want the 8. Compared to the 3, the 8 has 2 lower gears and only 1 taller (faster) gear, so it wouldn't gain you anything using the motor, but would gain you climbing ability on hills.