in relation to the Three Roots

...from The Light of Wisdom, Vol. 1, pg. 38:

As for "Three Roots", just as the root is the cause or basis for trees and flowers to grow, the guru and so forth are causes or roots for blessings and so on to occur. Numerous treasure root texts mention:

The lord guru is the root blessings.
The yidam deity is source of accomplishments. (siddhis)
Bless me fully, dakinis.

The Three Roots are therefore the especially excellent Precious Ones of Vajrayana. The guru's body is the Sangha, guru's speech is sacred Dharma, and guru's mind is Buddha. Peaceful, passionate, or wrathful--whatever forms the yidams manifest--their natures are sambhogakaya and nirmanakaya buddhas, and their tantras of Mantrayana are this extraordinary Dharma. The dakinis and Dharma protectors are the special sangha. Since the yidams and dakinis both appear from the guru's wisdom display, the guru is dignified with the title Lord of the Circle.