Need help in wiring boombox!!!!

I am making a boombox with this 2.1 60w amplifier which uses 12v x 2 more than 30w transformer i am planing to use 12vX3amp this transformer. I am using 3 stpd switches 1 for master on off , 1 for mains/battery selector, and one for aux /Bluetooth selector. 1 voltmeter two 12v batteries one dc input for battery charging and on mains input. Bluetooth i will be connecting with buck step down convertor. How should i do the wiring to the circuit. I really need help for this project. How much i connect the batteries? and the transformer is 12v 3 amp so which battery should i select and how should i connect spdt switch with mains and battery selector?

I am using this guide for instructables: 2.0 Portable Bluetooth Boombox - All

Any kind of help will be appreciated.

This is the circuit dig.
Thanks in advance!!
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