88 U can call easily vs most of the players both of the situations. So like a default gameplan versus unknows U can call with 88 any 3b versus any position vs a regular, reasonable size.

At the same time with 55 the answer most of the times no especially in the lower limits, because U loose more on nonSD with them, they have less eq versus most of the ranges, and also little bit harder to realize it when can hit more overcards the board then the case of 88.

At the theory standpoint every pocket pair has reasonable equity against 3b and U can defend them somewhat liberally, but be mindfull and carefull. It really depends on your skills, and the villains skills too. As people say sometimes to stuffs, bigger is better.

I've been playing 25NL recently, and most of the players I see are 3Betting about 8% of their hands. Most of that range is either a coin flip or dominates 88. I feel like calling is too loose as a basic set mining approach: call preflop and check/fold or check/raise if flop a set. Most of the time the opponents won't stack off, and when they do the stacks aren't deep enough to justify the rarity of hitting a set against the increased cost of the 3bet. Am I just nitting it up here? I'm more inclined to call if I'll be in position, but OOP, I feel like this might be a fold. Too nitty?

Fold oop you basically setmine and sometimes wont get paid off if you do hit

Ok. So let's assume what U said some player on the BU 3b 9bb to 3bb HJ open, the blinds are folded he 3B a range like top ~7% of hands (raw HU equity) like this:

MP3 40.48% 39.76% 0.71% { 88 }
BU 59.52% 58.81% 0.71% { 88+, ATs+, KQs, AJo+ }

As U see 88 against that range ha ~ 40% equity at the same time U need to call 6 to win 19,5 (like 1bb from BB, 0.5bb from SB, 9bb from BU, 3bb from your open and the 6bb what U call, thats the whole pot size after U call). So U need to ~30,8% equty to call if U can realize it in 100% of the time. Even if U can't just like ~77% of the time still an okay call. And thats not a polarized range, that vs a raw top ~7% of hands. Even if he only 3b for value with range of:

MP3 33.28% 33.09% 0.19% { 88 }
BU 66.72% 66.53% 0.19% { JJ+, AKs, AKo }

still have ~33% equity.

So yes EP vs MP 3B maybe it can be a reasonable fold sometimes maybe (especially lower limits with higher rake, but on the other hand vs weaker players in spots where U can realize your equity easier). But readless against BU 3Bs its a little bit nitty I think.

Finally try to let's have a look 88 vs a reasonable random BU 3B vs HJ range with some value portion and some stronger(tight) bluffs not too much from here and there.

MP3 43.77% 43.57% 0.20% { 88 }
BU 56.23% 56.04% 0.20% { JJ+, AKs, A9s, A5s-A4s, KJs, T9s, AKo, AJo }