Just checked the dealer portal for parts and the R3 and Thunderbird and Storm look the same but have different part numbers so it may or may not be a fit. I still have one on my accessories wall so you might try calling a few dealers to see if anyone has one left that they can send you.
The Thunderbird LT has a completely different seat and as far as I can see there isn't a backrest for it. What they call a "backrest" is actually a lumbar support insert in the seat. (#4 on the diagram) I wouldn't expect anything from the LT or the Commander to fit anything else.