"gainer" is a big enough category of cards that Gainer + Trail + Way of the Horse seems potentially useful.
That probably would have been true years ago, but as far as I'm concerned, it's starting to get a little questionable how useful even specific card + specific card combos are at this point. Not even just because they're rare (which they are), but because there's so many of them that you can't reasonably expect to memorize all of them — in other words, it is very frequently the case that you are playing a game that has a card interaction that nobody has posted about on the forum before, so nowadays you kind of just have to have the ability to spot those on the fly instead of relying on memorization.
In my opinion, memorization still makes sense for 2-card combos that work because of a non-obvious rules interaction (e.g. Trail + Way of the Mole) or because of a highly specific non-obvious way in which the thing just works out conveniently (e.g. Hermit + Market Square), and combos that are extremely strong and it is non-obvious how strong they are (e.g. Beggar + Guildhall). That actually excludes a lot of the stuff that was posted in the early days as exactly the kind of thing that was supposed to go into this thread, including a bunch of stuff I posted myself and basically all the example combos in the OP, but quite frankly I don't think it is an efficient enough usage of your mental resources anymore to see Stonemason in the kingdom and start wondering whether Quarry might also be present. I don't know what kind of a vision Adam has for this thread nowadays, though.
Anyway, the usefulness of specific card + specific card + card category was already officially questionable in 2015 and we've gotten, what, like 200 new cards since then? Seems pretty clear those don't count as useful under Adam's original intent.
Part of my dream was that these combos would appear in actual games of Dominion that were played. This probably means that they require only two cards to pull off. The odds of seeing three particular cards in a kingdom these days are extremely unlikely, and so they are much less useful. How much less useful? About 200 times less useful. Even requiring a village or additional draw or a category of cards to be useful is speculative.