If you are replacing TIU #3, then your new TIU should be set to TIU#3 before you do anything else. (Do it off the layout and with the other TIUs off.) Then swap your new TUI for the old TIU#3. Everything should pick up right where you left off, as the remote stores the engine assignments, not the TIU. The problem is the remote also associates an engine with a specific TIU address. When you replaced your #3 with a #1, that association was broken. So any engine previously entered that was associated with TIU #3 needs to be deleted and reentered. It will now be associated with TIU#1. BTDT.
Chris
LVHR