Connecting 3 12v batteries for a 24v system
You can't. Why not? There are 3 ways you could hook up the extra battery - in series with the first two, in parallel with both of them, or in parallel with one existing battery. Each has its problems:-


In series you will have 36V total - probably too much for the panels
and charge controller.
12V In parallel with 24V the voltages don't match - so expect lots of sparks and blown up batteries!
In parallel with one 12V battery the voltages match - but charging current will split between them and the other existing battery will get overcharged/discharged because it only has half the capacity.  


You should buy two additional batteries and wire them in series, then you will have enough voltage to put them in parallel with the original 24V battery. This will also give you two battery banks which can be isolated from each other if you need to work on one bank or use it for something else. 

When wiring batteries in series, try to use identical batteries which are the same age and condition. Buying a 'spare' battery to replace one that might die is probably not worthwhile.