Avalokiteshvara, Manjushri, and Vajrapani combined

[Tewi]

(This is a modified version of a question asked on the Nyingma page.) In Gelugpa circles, Tsongkhapa is said to be the embodiment of three deities: Avalokitshvara, Manjushri, and Vajrapani. I am wondering where this grouping came from, if it was not originally Gelugpa. I am aware of the tradition of the three dharma emperors being manifestations of these three deities, respectively, but lately I read that Sakya Monastery is said to have had one manifestation of Avalokiteshvara, seven of Manjushri, and one of Vajrapani. (I do not know who these refer to.) Perhaps this is the source, or one of them. Is there a Sakya tradition of grouping these three deities together?

[Aemilius]

In tibetan they are called rig sum ngonpo, Protectors of Three Families. They form a very old way of classifying the spiritual families or lineages (sanskr kula). They have an indian origin. It is generally thought that the number of familes have increased gradually: first three, then four familes, then five or six families and so on.. till one hundred families. They all exist already in Indian tantric buddhism.
Classifying people into different types of followers exists also in Hinayana and Mahayana Buddhism. We find for example in the theravada classic Path of Purification a description of three types of followers: Faith type, Wisdom type, and Body type (i.e. meditator or yogi type). You can see them as a parallel to the Three Family Protectors.