The latch at the front of the receiver that holds the front of the mag is wider on the CETME than the G3...
When people say that G3 mags will work in their CETME, what they are usually doing is jamming the mag in hard enough to deform (push it in enough) the front of the G3 mag enough to allow the back of the mag to lock in place.
While this will usually 'work', the G3 mags will eventually break if switched between an HK-91 and a CETME (because the front of the mag may need to be pushed back out slightly to be caught by the HK-91 'latch.'
If you'll only be using the G3 mags in your CETME, try mashing it in hard enough to have it catch. That should work...
Or, you can just pick up a couple of steel CETME mags. They are available at reasonable prices (but certainly not as cheap as G3 mags) and they'll last forever.
For what it's worth...
Forrest