Kaita

Kaita were fusions of three beings into one single being.

Toa Kaita
Toa Kaita were a fusion of three Toa. Once the beings fused, they turned into a completely different being with a different personality, Kanohi, and weapons.

Only two have been formed so far:
 * Akamai
 * Wairuha/Wairuha Nuva

Those two were the Toa Mata Kaita. Other Toa could form one, but just didn't. A Hordika couldn't form one since they lacked the mental focus to stay together.

Matoran Kaita
Matoran Kaita were formed when three Matoran fuse. As long as they have separate Elements, they can be formed.

None have appeared in the storyline, but two could be created from the McDonald's sets.

Turaga Kaita
A Turaga Kaita was a fused form of any Turaga of different Elements. Unlike the other Kaita, a Turaga Kaita required four Turaga to form.

None have appeared in the storyline, but one could be made from the Vakama, Nokama, Matau, and Nuju sets.

Rahkshi Kaita
The Rahkshi Kaita were fusions of three different Rahkshi. Only two have appeared so far:
 * Kaita Za
 * Kaita Vo

Bohrok Kaita
A Bohrok Kaita was a combination of three Bohrok into one. Unlike most Bohrok, these had two Krana. None have appeared, but two are known to eixist:
 * Kaita Ja
 * Kaita Za

Bohrok-Kal forms of these two could be created as well. Only Ja-Kal was ever formed.

Bohrok Va Kaita
The Bohrok-Va Kaita were fusions of three Bohrok-Va into one larger being. None have appeared in the story, but two were combiner sets.