I have a couple questions about memory banks in chirp. I have a TYT TH-UVF9 radio with memories numbered like this: A1 A2 ... A127 A128
B1 B2 ... B127 B128
Would this be considered two indexed banks with a size of 128? The same radio also has FM Radio memories 1-25. Would that be a third indexed bank?
No, banks in CHIRP are bins to which memories can be assigned, which doesn't match the concept in your radio. Your radio does it the way Kenwood radios do (which I suspect is a function of the fact that those radios all seem to stem from a stolen Kenwood design). I would recommend you just number the memories 1-256, and then use the "special memory" functionality to add the FM channels as FM01..FM25.
My Wouxun KG-UV3D also has FM Radio memories. How would I go about adding support for that to the existing chirp driver?
Same as above: add them as "special memories". Look at some of the more complete Icom drivers for examples.