The Three Lords constitute the second level of deity classification according to the Kriya Tantra system. At the top level of classification are the Three Buddhas: Shakyamuni, Amitabha, and Akshobhya who are the heads of the Three Buddha Families - Tatagata, Padma and Vajra. In the Kriya system there are as many as eight levels of deity classification for each of the Three Families: 1. Buddha of the Family, 2. Lord of the Family, 3. Mother of the Family, 4. Ushnisha of the Family, 5. Wrathful Deity of the Family, 6. Messengers of the Family, 7. Bodhisattvas of the Family, and 8. Nagas & Yakshas of the Family.
Note that the Three Lords: Manjushri, Avalokiteshvara and Vajrapani are not protector deities. In many non-Tibetan publications it is common to see the Three Lords mistakenly referred to as Protectors. The confusion is based on the Tibetan word 'gonpo' meaning 'lord' which is also used as a term for the class of Mahakala deities and others. The confusion also arises from Tibetan and Himalayan folk culture where a cairn is located at the entrance to a village and referred to as the shrine of the 'Gonpo Sum' - Three Lords (protectors). The village shrines most likely predate Buddhism and were originally unrelated to the Three Lords of Tantric Buddhism.