Is running two voltage regulators in parallel safe?
Do the math on your power budget:


The Pi needs up to 2.5 A @ 5 V = 12.5 W
The motor needs up to 3.0 A @ 12 V = 36 W


That's a total of 48.5 W

Taking the efficiency of the converters into account, which is going to be 90% or less, that becomes at least \$\frac{48.5 \text{ W}}{0.90} = 53.9 \text{ W}\$ that you need from the battery.

But you say that the battery can only supply 6 A @ 7.5 V = 45 W.

This will work only if you can tolerate overloading the battery each time the motor starts or stalls. (Motors generally pull their stall current when starting from a dead stop.)



And just to clear up the misconception in the title, the regulators are in parallel, not in series.