Yes, you can split the supply over the 3 motor inputs.
One suggestion is that you connect a separate power and ground wire from the connection on the supply to each of the three motor power inputs. This is sometimes called a "star" configuration and helps reduce the chances of the three motors introducing noise to each other's power.
Also, the voltage on our controllers is from 8V - 35V (see the schematic in the instruction sheet at
http://www.salemcontrols.com/product...C1125-3-MS.pdf), so the 5V supply might not be high enough voltage to drive the logic on the board.
Thanks,
Daniel