Which AGM battery terminals should be used when wiring 3 or more batteries in parallel?
When the wires behave as wires, then you can use any random positive and negative for the outputs. When the wires behave as resistors, then you need to think.

What's the difference between a wire being a wire, and being a resistor? It's the current being drawn, and the voltage drop that that creates across the wire. 

For a few mV or even 10s of mV (for instance 10A through 1m of 10mm2 wire gives 17mV drop), then use any random terminals.

If you have a significantly higher drop, then you should connect all your cells by equal lengths of cable to a star point, so that the load is shared equally between the batteries, and that one doesn't handle the lion's share.