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

yes;

I've used a 3x on a

1. Flatline
2. R-301

I know you said Assault rifle but, I've also used a 3x on a Spitfire.

No, I have not.

Not really,

I prefer a 1x or 2x Holo on my pistols and assault rifles.

But i use 3x on LMG's and sniper rifles tho.

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.

I liked using the 3x on ARs, LMGs, even SMGs, when I was less experienced with CQC and preferred medium range. It's a useful scope as it gives you good zoom while not taking too much of your peripheral vision. Lately though, I'm having troubles using it, maybe I need to fiddle with my mouse sensitivity for ADS. I prefer a 2x HCOG over the 3x at any moment, or a 2x-4x if I have the Spitfire, G7, Hemlok, Longbow or R-301.

I prefer the 2x scope over the 3x even on snipers. Zoom in on 3x takes too much time.

I used the 3x on the Spitfire sometimes. Actually id did not feel too bad.

I never even use the 2x scope on assult rifles, just the dot site, I just find it annoying. 3x scope is zoomed in way to much for an Auto gun I reckon.

If there's nothing else I'll put a 3x on the R301 or Hemlok because I can't really see with the iron sights, especially not with the Gold S2 skin. Flatline I usually just hipfire it until I find a 1x. 

3x WAS decent on the R301 when it had really linear recoil, hit a lot of long distance shots 75-150 damage, but with recoil changes bit it's not so great anymore. I'd assume the 3x would pick up in popularity with the start of this new season with that new Anvil Receiver hop-up.

3x single fire yes. Flatline 3x lands more headshots. Hemlock 3x contests snipers, 301/flatline new hop-ups will be good with 3x