Yes, we actually have enough information in the books to deduce all but one of their OWL subjects.
Charms – in Order of the Phoenix, both Fred and George mention having a lesson inspected by Umbridge. Since Order of the Phoenix would be their seventh year at Hogwarts (so NEWT year), this means they must both have taken (and passed) Charms OWL.
It’s clear that Flitwick was impressed with both of their work — when their Portable Swamp appears in the castle, he leaves a portion of it intact as a testament to their ability (and propensity for mischief).
Defence Against the Dark Arts — in Order of the Phoenix, the pair get told off for playing Exploding Snap in Umbridge’s class. Again, this is NEWT year, so they must both have passed DADA OWL.
Transfiguration – in Goblet of Fire (iirc), Professor McGonagall tells George to continue transfiguring his raccoon. Since this is the year after he would have finished OWLs, we can assume that he passed Transfiguration OWL also.
His ability for transfiguration is shown from an early age — consider, for example, how he turned Ron’s teddy bear into a spider at age five.
This gives us three for George, but the final OWL for Fred is unaccounted for. His OWLs are never named in the books, but some people believe his third OWL was Herbology. In Order of the Phoenix, the twins are going to sell Extendable Ears before Herbology. This suggests that at least one of them is attending Herbology classes, and since it isn’t George, as he is already in three other NEWT classes, Fred seems like the natural fit.
I don’t have any information about which subjects they took and failed. You could probably read the first three books to find out which subjects they mention for OWL, but I don’t think we’d learn much from that.
So two of your three guesses were right. As for Potions, we know that Snape wouldn’t have accepted them for NEWT unless they got Outstanding (which is probably pretty unlikely), so I think they probably didn’t bother studying. Indeed, I’d guess that the pair picked these subjects purposefully, because it would have given them plenty of time in sixth and seventh years for… “other” endeavours.