Tsutama was a Sitter 20 years ago, I'm pretty sure she isn't one now. I have no idea who is the third Red Sitter, though I do recall that Pevara and Javindhra are Sitters.

Hmmm..Katerine brought the news of the Dumai's well catastrophe to the Tower in CoT, I think. That's when the Reds would have realized that Galina wasn't coming back and would've chosen a new Highest, I think. Meaning that it's possible Mesaana is Tsutama. Not that I think Mesaana would ever put herself into such a responsible position. Much better to be a Brown or White, since that way she can do whatever she wishes and not be a part of the daily life of the Tower. As the Highest she would be responsible for personally ruling the largest ajah the Tower has. Surely Mesaana would prefer to remain as the power behind Tsutama's jewel-encrusted chair.