Do you have Universal Email applied to the register3.asp page? Where is the Universal Email folder in relation to this page?
If you have the UE folder in the root then you should try starting the include line out with a slash like this:
If it is in a sub directory from the root you could try adding in the directory to the this path like this:
If it exists in the same directory then you could leave the slash off completely and it should find the file correctly.