Hi guys,
I will be purchasing the WOFFUE and I would like to know if I should install it over my existing WOFF3 or remove WOFF3 and do a clean install of WOFFUE?

You can do it either way.

See section 2 in the FAQ file. http://www.overflandersfields.com/support.html

Before installing Woff use the backup tool as can be found on http://www.thatoneplease.co/indexWOFF.html

Although it can install with WOFF3 there, I'd recommend a clean install, you may have mods or other changes in the old WOFF.
See section 17 of the WOFF UE FAQ if you need to remove first.