My initial plan was a red dot with 3x flip to side magnifier but the claw mount on the g3 isn't long enough for both.
The very cheap <£10 flippy magnifiers are for novelty purposes only. I bought one just to see how bad they are, and... bad. Essentially non usable.
Airsoft optics, and Chinesium claims. I took another punt on a ~£20 "3x42" red dot sight like this:
View attachment 63878
Nah, I don't think so. Maybe 2x35 on a good day, with more red glare than an Amsterdam window. Just about better than Mk1 eyeball, depending on lighting.
What does work for me is a 3-9x50E illuminated cross sniper scope, whether it's on a bolt action, or an AEG. As noted, it's as much about seeing where the BBs are going as spotting the target, although I did throw it on my stubby M4 right at the end of my last game as the light was just starting to fade and the Bush Wookies were having a field day.
View attachment 63880
A bit heavy, but effective. I'd say 3x to 4x is a sweet spot. Above 4x the FOV gets too narrow. You'd probably be fine with a 40mm objective lens but I like the extra light-sucky-in-ness of 50mm.
I'm not sure about the sights on the G3, but on a G36 the stock sights are low enough to remain just about usable with see-though scope mounts as above, of if you're going for a more solid sight, on top of a hollow rail riser like this.
View attachment 63877
Although as
@Lozart notes, with a DMR if your too close to use the optic, you're probably too close to shoot.
As with much of airsoft, it's a case of suck it and see what works for you, and there's no need to be bound by convention, or reality. I've run that 3-9x50 scope on an MP5K for larks.