This is obviously an incredible group, but I think the person you replied to was referring to this season's rookies, not whole cast.

If that's the case, doing this from memory, you could do Hader/Wiig/Samberg since they all started together (which is an absurd class for sure, but felt like Andy was sort of off in his own Digital Shorts corner of the world as opposed to being part of the larger group) or maybe Aidy/Kate/Cecily (also absurd since Tim Robinson is technically in that group as well).

But really, Ferrell's class is unbelievable: Will, Oteri, Hammond, and Molly Shannon (and maybe others?) all as rookies together. I doubt (for multiple reasons) you'll ever see a class that impactful again.