Multi battery setups for big audio

I want to wire up 2 or 3 batteries to power an audio system I plan on building. Does anyone have advice on a proper system, and know of the necessary components to incorporate them into electrical system? I will go with agm batteries specifically for audio, and install them in a truck box in bed. My starting battery is an odyssey extreme.

 

Blue Sea ACR will do the trick.

https://www.bluesea.com/products/7620/ML-ACR_Automatic_Charging_Relay_-_12V_DC_500A

Also use Blue Sea's calculator for proper wire size for your needed amount of system AMP draw and length of wire used.

http://circuitwizard.bluesea.com/#

You can reference my build for some photos to get an idea but I did dual batteries.

https://www.tundras.com/threads/dans-2014-mid-travel-tundra-build.24083/page-20#post-1557374

 

Here's Blue Sea's digram for 3 batteries

https://www.bluesea.com/systems/41/3_Battery_Bank_1_Engine_Electrical_System

 

Pretty extravagant for what I need. At some point I'd like to build this but my goal right now only concerns audio. Its a good illustration as to what's required though. Thanks for that.