The "Both" position should put all three batteries in parallel for whichever switch (engine) is in "Both." This could be used if either of the banks are weak and three batteries would be needed to start an engine. Think of the switch being permanently assigned to either port or starboard engine. Starboard switch to starboard engine, port switch to port engine. You can connect either engine to either bank of batteries/battery or both. For example, if both switches are selected to ,say, #1 battery bank, then both engines would be started from #1 bank and would charge the #1 bank, not necessarily a good idea to have two different systems charging the same battery bank. Might not matter with more modern charging systems, but I don't do it. Also remember to never go thru "OFF" when changing selections with engines running as disconnecting all batteries from the engine charging system might damage the charging system. It is OK to change selections from 1 to 2 or both as the switches are "make before break" type switches which never go to off in between active selections (1,2, or both) If all of you batteries are kept in good condition the "Both" position is seldom if ever used. Hope that helps.
