I have 2 12v batteries wired in series to get 24v. Can I add the third battery to one of the other two in parallell?
No this is not a viable idea at all.
If you have three batteries, there are only two ways to arrange them. All in series (3S) or all in parallel (3P).
If you want to add capacity to your battery bank, you should add two more batteries so that you have two in parallel and two in series (2P2S).
There are many details to see to when it comes to combining batteries in series or parallel, but you can read up on that separately. That is not what you asked about.
The reason you don't want to have 2P in series with 1P is that the 2P acts like a single battery with double capacity.
For example, if you have 3 batteries, 2Ah each, and you put two of them in parallel, those two effectively become a single 4Ah battery. Now you put that 4Ah battery in series with a 2Ah battery, and you have problems. As the pack discharges, the 2Ah battery will reach end of discharge first, and at that point, the 4Ah battery will still be 50% full. This is a bad state of affairs and is essentially never done. If you cycle a battery like that many times, the 2Ah cell will wear out much faster and reach end of life much sooner.
In general, don't combine cells at different life stages. If you have 4 brand new cells and want to combine them 2S2P, go ahead (but do some research first to make sure you do it the right way).