The interesting point here is the omission of Rudra, the third of the Divine Triad. If you are talking of the divine cosmic functions, the triad must include Rudra, certainly. But the purpose here is different. It is to show the supremacy of Mother Goddess over everything.
The three divines mentioned here are, Indra, the King who maintains the divine world, Vishnu, the One who maintains the entire world, and Brahma, the One Creator to whom all the divines and all the universe make their first appeal, whenever there is a universal crisis. So the maintenance of the created world is the thing in question here.