With the recent gold funneling strategy becoming popular in 5v5 to the point it's being nerfed and constantly looked at, it feels like a pretty big slap in the face of 3v3 players.
This sort of strategy has been present in 3v3 for a really long time now, yet nothing's been done about it. The concept is the same, one player picks a "hyper carry" champion who clears the jungle and one lane, the other a support while the remaining player(s) tend(s) to their lane(s).
Though it's a bit different in 3v3 due to difference in map, jungle camps and lanes, the "cheese" is about the same. And the same reasons RIOT stated for wanting to change it in 5v5 have been present in 3v3 as well. You feed one champion up beyond comprehension who proceeds to try and solo carry the game. Against people running the classic Top-Jungle-Bot composition, you have high odds of winning because you've got 1 person who pops off and is under constant protection from the support, and proceed to steamroll the game most of the time.
Luckily, there's a lot more "viable" picks for such strategies in 3v3, so both teams *tend* to have a hyper carry and support, making it somewhat more fair, but still redundant.
It makes bot lane a joke too, since you're under a lot less pressure from the enemy jungler. Unless you play really bad, you most likely won't get ganked by the duo, especially with a little bit of communication; whereas in "regular" 3v3 games, you have to play carefully not to get ganked by the jungler, especially those who have a way of getting through/over walls (Zac, Rek'Sai, Kayn come to mind). And it feels like most of the time you lose, their hyper comp was simply better, while if you win, your hyper comp steamrolled theirs.
To summarize, the meta's been stale and present in 3v3 for a long time without any changes whatsoever, while in 5v5 it's been directly changed almost straightaway, and is being kept an eye on. There's plenty of reasons as to why people don't play 3v3, but stale meta and almost no attention from RIOT have to be one of the main reasons.