Those pumps have a about a 15A continuous and 20A spike. Most boats use a Carling Tech Contura rocker body. They have a 20A rating, so they will handle the current of 1 pump. They need to be a 6-pin Double Pole Double Throw (no illumination) or a 7-pin DPDT if you want the switch to light when the pump is running. So each pump needs to be wired to its own switch, unless you do the 3-relay per/pump configuration and use a single SPDT to trigger all 3 relay clusters at once. I would go with individual DPDT switches for each pump and ship the complex rely setup.

Your roll of 12ga will work fine. Just down forget to fuse each switch. Also, you need to make sure that the source you draw from, if it happens to be the helm BUSS, is able to handle the additional 50A current draw you are adding to it. If not, then you will need to run a pair of larger ga feed wires directly from the battery to the helm for the 3 switches.

You will need at least a 1.25" thru-hull setup if you plan to run all 3 pumps at once.