Above 100bbs - x3
100bbs - x2.5
60-40bbs - x2.3
30-20bbs - x2.15
Under 20bbs - open limp your entire range. Fold the cards you don’t want to reshove. I usually never limp call
If the players in the blinds are defending a lot I’d size a little larger. If you are raising over limpers then use the size and add 1bb per limper. If I’m sitting on 100 plus bbs and everyone else has less, then I use the size that fits the table. If I’m in the BB and the SB limps then I usually re raise x4. If it folds around and I’m raising from the SB I use the size appropriate plus 1bb. Hope that makes sense. This is for mtts. In cash games it’s good to mix your sizings pre
Edit: I read your post again and I’m sorry if I mis read your op. But it’s good to have a set of ranges that you 3-bet from all positions against all the others positions. Have these ranges for the different amount of bbs you and the pfa has.