I was wondering if any players had any tips for constructing 3 betting ranges oop (blinds and ep). I know it sounds elementary, but I've been struggling with that as of late. Pretty big leak imo

For Ex:
1/3 NL
UTG opens to 12, there are 4 callers. This dynamic had been happening all night in general. I have AJo in sb it's either 3bet or fold. I know that flatting is out of the question. What sizing am I supposed to use here? Is 50 good? I guess my fear in doing so is having everyone flat my 3bet and having no idea where I am in the hand, because as soon as the UTG player flats it incentivizes others to do so as well. This particular hand I folded, simply because I wasn't comfortable with 3 betting.

What hands do you choose to use as a bluffs to balance out your stronger holdings(AJ+, big pairs, etc.)

In terms of player types lets just assume we've sat down and we have no reads on players. Any insight would be helpful.