Finally got my first UMP-45.

Considering how difficult it was to get my first one, with a G36c dropping earlier than her, should I invest some cores to get her to x3 immediately or should I hope to get her a few more times?

So I got a Thompson too.

With her I finally have 4 decent SMGs , G36C, UMP45 and JS9 (from event points).

As far as ARs, I’ve been using the ones the campaign gives me. AR team is now level 70+ and x4.

Should I throw cores at the SMGs now?

UMP 45 > G36c > JS9 > Thompson

UMP 45 is great.
JS9 is a flex option, she can either main- or off-tank.
G36c is the best of the forceshield SMGs, since she has decent evasion while not invincible.
Thompson will be OK early on, but later her stat spread (low evasion, high HP) will fall behind.

+Eva, -FP exos on maintanks always. Make sure they’re leveled. Make sure you level skills.