The only way to guarantee 3 pups is if either the male has maxed charisma or the female has maxed maternal, right?

Not really. You'll most likely get 3 pups as long as your female has like, 60 maternal or the male has 60 charisma. It doesnt NEED to be maxed to get 3 pups, but if you have maxed maternal or charisma, you are guaranteed 3 pups

Thanks. So is likely that I'd get 3 pups with 60 maternal or charisma, but to get a 100% chance of getting 3 pups, would I need one of those maxed?

Basically, yeah. I always try to max my maternal just to be safe, but it really doesnt need to be maxed. I can usually get 3 pups from around 60 maternal

Thank you :)

I almost always get 3 pups with only 40 maternal. 50 is a safer bet but it's enough.