Add on 3rd battery
 

I have a 91 290 baja with twin 454's. Has 2 batteries, 2 perko switches. Was wanting to add a 3rd battery for stereo system and be able to be charged with the alternators like the other 2. Anyone got a drawing showing how to go about doing this?? Is this done?
Also since I have owned boat, this dual battery, dual switch set up, has not allowed me to say use the port battery to crank starb engine. Thought that was the reason for the 2 perko switches. Buddy of mine has same boat, year newer and his is the same. Can anyone help??
Thanks in advance.