I don't see a reason why it can't be a balanced 3, 3, 3 class party system either. However, just having a companion number being divisible by the number of available classes isn't compelling evidence that it will be even either.
In DA: O, we had 3 warriors (Alistair/Loghain, Sten, Oghen), 2 rogues (Leliana and Zevran, and 2 mages (Morrigan and Wynne) plus two melee "beast" types in Dog and Shale. If we don't include those two as a typical class, it's a 3/2/2 ratio. However, I've seen the argument that they function as warriors, which would shift the number to a 5/2/2 ratio.
In DA 2, we had 2-3 warriors (Aveline and Fenris with Carver optional), 3 rogues (Varric, Isabella, and Sebastian), and 2-3 mages (Anders and Merrill with Bethany optional). That means that we have either 3/3/2 or 2/3/3 ratio.
It's roughly equal, but we've never had a perfectly equal ratio, so I'm not convinced that the remaining three companions are going to be one of each class. It might turn out that way, but I'm not going to use that as fuel for my speculation.
I agree that we have (effectively) 6 known companions:
Cass (female human warrior)
Varric (male dwarf rogue)
Vivienne (female human mage)
"Warden" (male human warrior)
"Sera" (female elf rogue)
"Baldy (male elf mage)
There are several options for the remaining three spots right now:
Cullen
Iron Bull
Moustacio
Cole
Dorian
Clearly some of them aren't going to be companions. Some might. Who knows, we might have three more companions that we've never seen any footage or concept art of up to this point. What I find more interesting is that we don't have any more females that are even on the table. So we are either going to have a really disproportionate male-female ratio (6-3, which is not terribly unlikely) or we will start seeing some more concept art/footage of other ladies soon.