Has anyone ever used a 3x scope on an Assault rifle ?

Yes, if that's my only long range weapon. I'd take it off, when I'm running into a cqc fight and put it back on when traversing in the open.

Although, since I'm usually lazy, I would generally use the 2-4x if I had the choice between that and a 3x (since 2x is somewhat usable in CQC, when I'm too lazy to swap scopes and 4x gives the range), but the 3x is definitely much better sight overall (clearer, smaller dot, much more open and less covering). It's still better to take the 2x off in a cqc situation, so if I was going for max performance, I would definitely go for the 3x/iron sights option.

It's a pity when I see a 200m easy kill and all I have a is a 1x on my R301 - sure, still doable, but 3x is sooo much better for that.

P.S. I would generally always keep my primary weapon with a 1x. The only exception would be when the secondary is a p2020 (then the 3x/1x swap is the optimal solution even for the primary).

P.P.S. If you never played PubG, then you may not be as used to constantly swapping scopes.