I also have three pumps and I ran mine off the Perko switch, this will not allow the pumps to turn on unless battery is turned on. I also used a 30amp inline fuse for each pump with no distribution block. I have instaled a few different Johnson pumps on different boats using this connection with no problems. To run off the Perko its very simple, run the hot off the Perko and ground off the battery. If you do not have a Perko switch I cant see why you just could not come directly off the battery with inline fuse.
I use the Romex wire from Wakemakers, its really clean and easy to feed through areas of the boat. I believe its 12 gauge, but they also offer 16 gauge.