3 genning isnt an issue
...its much more an issue with some design choices. On a balanced map, against your average killer, simply splitting up on gens will eventually result in them popping . The real issue is small maps (namely dead dawg) and zoning area control killers.
Skull merchants 3gen is damn near unbreakable in soloQ due to the free info and quick downs. Her kit is practically tailored towards 3 genning; providing very strong chase for those inside the zones, a speed up to patrol and down faster, information to know which gens to focus, and encouraging survivors to either stay injured or get wallhacked. All of that together makes for a very powerful 3 gen that uncoordinated (soloQ) teams cant handle.
Smaller maps and just general generator spawns are also a big issue. If im on dead dawg, odds are theres a 3gen I can patrol in about 5 seconds (if patrol = establish LOS to each gen). Coordinated teams can still do it, but at the cost of many, MANY hooks. A killer whos been useless all game can easily turn a 0k into a 1-2k with a good dead dawg 3gen on a strong killer.
TL;DR rework gen spawns and skull merchant/knight. Outside of that 3 genning is just a non issue.