IMO its best to just swap out the whole bolt/carrier of the same type. If you plan on using an 8mm type (which works fine in .308), I'd use the 8mm type locking wedge and 8mm type carrier.
I have not tried a MG3 type bolt carrier with a 8mm type bolt head. But I do know the locking wedges differ (their actual name escapes me right now - maybe our friend John can help out out on the term?
Is it called a striker perhaps?
I have a both 8mm type (M53) and 7.62 type (MG3) wedges and the rear "tail area" of the locking wedge are different diameters, with the MG3 type being much thicker in dia., so the wedges differ as well.
On the bolt heads their difference are mainly they use a different extractor spring and keeper/plunger
...the 8mm type plunger/keeper is square, the MG3 type round. The extractor springs differ as well, though the extractors themselves are the same
and can be switched between types.
I do not have an MG3 carrier to tell if they interchange though, maybe someone else might. But really I see no advantage of combine the type and I would simply use the 8mm type if the that is the bolt head you have long with it wedge and carrier.
Keep us posted on how you make out because I'd like to know the answer to this questions as well. Hope this helps you out some.