This is why I feel like 2 Toa forming a Kaita would be better:
The Kaita never got a lot of screen time in G1, because they didn't exactly have a lot of character. Trying to do a three way mix of personality would just be over complicated and would almost guarantee some conflicting characteristics.
That's why I think there should be two Toa per Kaita. This way you'd only have to meld two personalities, and it'd be a lot less difficult.
I also think this would make the Kaita much more practical in a story telling sense. When the Toa become Kaita there would still be three of them, and there could be a much more interesting group dynamic.
It could also be used as a way to explore the 1 on 1 relationships between the Toa, and would make "Unity" more interesting then the Deus Ex Machina it was in G1.
The pairings are pretty obvious as well.
Tahli Toa of Leadership: Tahu and Gali. They have to find the balance between control and instinct, through unity.
Onewa Toa of WillpowerOnua and Lewa. They have to find the balance between light heartedness and seriousness, though unity.
Pohaka Toa of HonorKopaka and Pohatu. They must find the balance between cold determination and outgoing relentlessness, through unity.