I just sat down and don't know any of the players. There are only 6 of us at the table. I open the pot for $35. Everyone folds to the SB and SB 3 bets to $125. He has $800 total behind and I have him covered. Should I call/fold/push all in?

please help/let me know

You opened much larger than standard (unless the whole table was doing 7x opens. I usually see $20-25 at 5/5), so I'd think sb's 3b tighter than usual, but there's no way I'd fold. Def just going to take a flop here.

I think a few google searches on PF ranges would help you out. I think upswing has some free ranges and CLP has a whole hourlong+ video on youtube all about pf play done by Ki Lee. I'd check those out. Ideally, you want all this worked out before sitting down at the table.

Obviously not folding vs an unknown at 5/5. The choice between flat calling and committing with a 4 bet is more up in the air. With position on villain I don't mind a flat call but a 4 bet is OK also. This being unknown vs unknown means your 4 bet will put a huge amount of pressure on villain.

If you do flat call plan to call a flop c-bet on all but the worst flops and then evaluate further on turn.

Your preflop size is too big, 4X/5X is fine in general at 2/5 and higher. The odd bet sizing may have induced some odd action from villain but without knowing something about villain can't tell how he is reacting. If he thinks your just trying to take the blinds or have some JJ/TT that doesn't want to see a flop he could be pressuring you wide. If he thinks your trying to build a big pot with a good hand he could be playing tighter then usual.

Even having just sat down, there's some potential info conveyed by the physical appearance, etc. of the V based on tendencies - What can you tell us about V in the SB? But without that info, and probably even with that info, think this is a call and evaluate, especially playing in position.

What was your position, were you UTG?

What's the max buy in, is he under max? (more recs play below max than winning players, which tend to tighten their ranges up alil. If max is 1K he's more likely to have a big pair and adding in he's in the sb makes it easier to just flat pre.).

You have position, which would probably be the safest way to proceed, since if you raise now, you can't fold to a jam because too much of your stack would already be in the pot.

I've looked at people thinking they're tight recs and flat then later find out it's just a middle aged guy who likes to gamble with random junk hands, and I've raised young winning pro's with egos and they turn out to have aces but without knowing more, there's nothing wrong with flatting.

edit: didn't see MP in the title

Call, play a flop