[+ I'd like to add a resistor between source and ground.]
problem: it works [for a long period] without the 1k resistor, with it doesn't do anything.
Do I need 3 separate power supplies
Do I need to make a star grounding for these 3 motorcircuits?
does it need a piece of metal to drain?
is it smart to use the same Arduino as for the motors for this?
Is a thin wire from 1 Arduino/circuit/point reasonable?
No if the one will supply all the current then you can use that. How much current will each motor take?
hmm .. a non logic level won't do?