No chance for TLR-3 keys to work...
Way too much material on the bottom of the TLR-3 keys. The older design has the cross screw completely inside the key... while the new one just goes over it. There are also two tabs in the new keys, opposed to a solid sled of polymer compared to the old ones.
TLR-7 with its key (UNIV-1, which I use with my Glock 30S.
TLR-7 with a TLR-3 key (believe it was the B key... C is in my TLR-3).
And a shot from the rear of both lights with their keys in. Can see how much higher the rails are in relation to the key/cross screw on the TLR-3.
I don’t think you could even modify a key to work. Probably would be a better route to talk to the guy who printed the other HK keys for the TLR-7/8, and see if he can help you get something. Even going from scratch is probably more effective that trying to make it work.