Simple questions about Battery network
What you have drawn will work, but the effective capacities are not what you show.

The 4.5V supply will be limited in capacity to that of the 1.5V batteries, or 300 mAh total. At that point, the 1.5V batteries will be depleted, and you'll no longer get 4.5V.

Furthermore, this will also consume 300 mAh of the capacity of the 3V batteries, which means that you'll only have 900 mAh of capacity remaining to run the motors at 3V.