Idea for solving 3-gen
A lot of people bring up gen placement and map design, but I think there's a way to address it without changing maps. When there's one gen remaining, generators can no longer regress passively. The killer can still kick generators and get the 2.5% regression, but beyond that generators won't regress on their own (Hex Ruin would be the exception). This would nerf Call of Brine and Overcharge, as the increased passive gen regression wouldn't be in effect at this point in the match. Since they were originally nerfed because of 3 gens, they could probably be buffed back to where they were or close to it.
Jolt/Surge, Eruption and PGtW would probably need some form of nerf as well. Surge and Eruption could have a short cooldown (maybe 20-30 seconds). For PGtW, maybe make it so it no longer activates after hooking a survivor for the 3rd time/death hook (reduces uses from 12 to 8). At one gen remaining, there would probably be at least one or multiple survivors on death hook, so PGtW would get less opportunity to be used in 3 gen scenarios.
These changes should be fairly easily to implement. Some of the typical 3-gen builds/perks could then be buffed to help with slowdown early-mid game since they will lose their passive regression effect when only 1 gen is remaining (Call of Brine, Overcharge, Oppression).