is all types of 3 genning bad?

There are situations where it isn't bad. If it's been a hard fought match, and the killer is backed against the wall. The survivors have accidentally 3 genned themselves and the killer really has no pressure. That is understandable. Three genning is bad when killers are designed to be masters of it. Three genning is bad when the killer prepares for it at the beginning of the match, refuses to commit to chases and intentionally makes the match take ages to complete.l

3 genning as a strategy isn't really the major issue. It's actually a normal consequence of standard DBD gameplay.

The major issue is that the game has no natural cut off for how long it can be maintained. It lasts as long as either side fails to be or decides to not be proactive enough to break the stalemate. The fact that recent killer designs have been largely relegated to using this playstyle as it is the most effective way to use their power makes the situation worse.

Anything's tolerable for around 2-5mins, but nothing is tolerable after 45mins of making zero progress one way or the other. Trapper and Hag can't drag a game down to the degree of notorious 3 gen killers. Trapper isn't able to spend the time and resources to required to lock down an area so thoroughly. He can cut off a couple of pathways at most, but he still needs to do something else to completely stop progress. Hag's a bit better with her not needing to pick up and go through time wasting animations with her traps. Still he traps are so easy to play around at this point that she isn't going to totally break up the flow of the game.

DBD isn't really built in a way where such strong area denial powers like Skull Merchant and Knight are healthy. It would be on thing if survivors had either auxiliary objectives that could break stalemates or if the time limit of trials was capped to be around the 20-25min mark at max. It would place a sense of urgency on both sides to get things over with rather than drawing the game out. In a battle of attrition in this game the killer should never lose, but the game shouldn't be a battle of attrition. 1 side needs to win or both sides need to lose/draw in shortish order. 20-25mins is more than enough time to get full BP, have plenty of chases, get gens done, etc. and if neither side has met their win condition the Entity loses interest in the trial lore wise and game mechanic wise you're scored on what you did do and sent off to the next game.

more people might play into speedy hag now since the addons getting a buff. that her only hope

Well, soon most SM games will likely involve at least one bot, and bots have no game sense, and can't strategize or coordinate.

I suspect these 3-gen skull merchant games will just devolve into killer vs 4 bots after the first person disconnects.