You can swap an MG42 over to MG3 "status" by swapping barrels, boosters/flash hiders feed tray and top cover.
The
MG3 also uses a modified bolt that runs slower and has a few
improvements over the MG42 bolt as well. The buffer is different and
well as the recuperator, though parts will interchange. The MG3 also has
a built in flip up AA sight as opposed to the MG42 spider sight.
One big improvement to is the top cover and feed tray is made for not
only 7.62 nato, but the MG3 top cover can run DM1 belts, M13 (M60) links
as well as improved DM6 links. The MG3 top cover also has a spring that
keeps the cover from falling on your fingers when opened and the MG3
feedtray has a tab that holds the belt better.
I'm sure there are other differences, but those are all I can think of off the top of my head.
I have a M53 (MG42 clone) BTW that is currently running many MG3 parts: