I'm replacing my shower with a Moen system that has a Moentrol three function transfer valve Model 3330. The inlet and outlets are copper to copper. Other than removing the cartridges prior to sweating is their any particular trick to heating those connections on the valve? Thanks very much for your help.

Emery cloth outside, Wire brush inside, flux, put pieces together, Apply heat to opposite side and have heat of copper to draw in solder, You don't want to melt solder with torch.
Wipe clean any excess.
Also remove any water from pipes or connections.