Re: perko switch ?
Technically, yes. However, if one is a starting battery and the other two are deep cycles (typical trolling motor setup) then you should get something like a ProMariner tripple battery 20amp onboard charger so the deep cycles are on their own independent charging cirucuits. The risk is that one or more batteries may be demanding a higher charge rate and you'll cook any that are not. You can get an isolator from Perko that will isolate the batteries during charging but the tripple isolator is nearly as expensive as the ProMariner.
Yes, I realize you said you have a trickle charger. That means a charge rate of 2amps and with 3 batteries it will take you forever to get them all up to full.