How to wire 3 x 12 V batteries to increase both voltage and amperage?
With those three batteries, and a requirement of 24 volts, I'd use the two 125 Ah batteries in series, and ignore the 120 Ah. That will give you 125 Ah at 24 volts.

There is no sensible way to use all three 12 volt batteries to produce 24 volts.

Edit:
Preferably, all batteries in a bank should be of the same make, type and usage history to ensure that they all share the load equally, and age at the same rate. 

If your batteries are all fairly new, I'd put the two 125 Ah in series, and try to get a 120 AH to put in series with the existing 120 Ah.