hey guys - this hand happened today against an aggressive opponent. he check raise shoves the flop 2 diamonds and an ace, having called my raise pre flop. i felt like he was protecting an ace against 2 diamonds, or perhaps a set. would love you thoughts if this was a bit of a nitty fold

Raise more pre, check flop

Not nitty at all. The fact that the Ad and Qd are accounted for is highly significant here as they block most of the fds villain could do this with [AdKd, AdQd, KdQd]. Your hand was super strong pre but now it's only 2nd pair.
Villain can have a set but from a hand reading perspective AA is unlikely [4bet pre] and 22 is unlikely [fold pre].

This is a weird line from villain that seems to scream set of sixes, but could also be fishy players doing it with AQ and AJ as I have seen a lot. AA and AK are most likely 4-betting this pre out of position like this, but 66 and maaayyybbbeee 22 would probably just flat and would thus be the best candidates for check raises. KQdd is not available as a check raise as you already have the Q of diamonds so unless he 2B/flatted 45dd out position and is playing an inside straight draw + flush draw like a maniac, there just isn't any reason to call here.

Multiway, it's a clear check back on the flop. What is the purpose of your bet? You're hardly ever getting called by worse (villains will put you AK, and only call with hands that have good equity against it), and you're not making a better hand fold. Take the free card, so that you have an opportunity to bink a 2-outer or pick up the seond nut flush draw, which will allow you to get to showdown and win more often (you can beat some turn bluffs, or hands that make a crappy second pair).

Hi! So the first mistake you make is your 3bet sizing, 3bets are great because they usually offer more clarity about V's range. A standard 3bet is usually x3-x4 the size of the open raise. The problem with this sizing is that not only UTG is getting goods odds to continue. The BB is still getting good odds for a lot of hands and him joining is going to decrease our equity. Also FYI: A standard 4bet is usually x2-2.5 and the same holds for a 5bet. Ofcourse if you're short-stacked, and risk a very low SPR it is probably best to shove as a 3/4/5bet.

The second mistake you make is c-betting this board. Before you c-bet ask yourself: Is it clear what I have to do if I face a raise? That way you don't get into these "ugggh" situations. Another reason why we don't want to c-bet here is because there are 3 people in the pot, mathmatically speaking we would need an 33% equity edge to raise for value, and I really don't that happening against a c/r range. Even if we were heads-up we wouldn't want to bet since we don't need a lot of protection (we have the Qd). As ArtyMcFly also mentioned we want to see a free card and play some turns.

My main line would be to 3bet to 700. Check the flop behind. Call a bet OTT and fold to a bet OTR (depends on sizing though) if we haven't improved.

Good fold.

you will learn alot from the forums welcome here.
as you can see even the beginner forum has very good if not great players responding to your post. the quality is very high.

you need to learn about bet sizing , meaning when you make a certain sized bet just what are you trying to accomplish with that bet. that bet size doesnt make much sense unless you thought you wanted it checked to you on the flop. or believed it may drive out all players or get it headup. which it didnt because it wasnt large enough.

then learn about hand value after looking at the board not just preflop as hand values change every time another card comes out. here you should have checked on the flop as the ace is the card most likely to be in anothers hand. your hope would be to run out the hand without putting any more money in the pot.

yeah you wanna milk as much as possible pre. a bit bigger bet should be enough to get a call with.

3 way pot and you have middle pair with a Ace on the board you wanna be checking the flop.

Basically - thats it.

the re- raise size in a pot has to be in relation of how your planning to play the hand out against that opponent. its not a standard thing.
plus stack sizes are taken into consideration.