I have three Roku TV's at my bar. When I attempt to only change volume channel etc on one TV all three TV's react. Is there an aftermarket remote or way to customize the remote for only the intended TV to react at a time?
There is no way to do this. You either have to buy a new TV (with a new remote) or... wait there is no other way*. All roku TVs use the same remote, and all roku boxes use the same remote. Luckily, Roku boxes and Roku TVs use different remotes, so they can be used in the same room.
*I lied...there is a very impracticable way to do so involving different wifi setups and remotes, but in the long run it would be cheaper getting a new TV. The only short-run solution is get REALLY close to the IR with the remote, and hope it doesn't affect another device.