Who might the other two SMCs that most wanderers on Earth incarnate from be?

[Nía]

I have a lot of resonance with the notion that the SMC that helped out South America was Quetzalcoatl.

I think you'd need a pretty clear channel/channeling to find out for sure though, because after all, who knows?

Quetzalcoatl = Q'uo

What makes you say that?

Names are distorted by the channel and their social expectations of what a name should be, which is to be honest, the smallest of distortions, not extremely important, so I imagine the entities in question simply "roll" with what their assigned/translated name becomes.

Actually, in their very first channeling, Q'uo hinted their real name was longer than Q'uo:

We are called Q’uo. This instrument does have but a portion of the name we are attempting to transmit, however the naming is never important.