The last goddess of the trinity is Saraswati, the wife of Brahma. She is a gentle goddess of art, music, and knowledge. One meditates on her and prays to her for help with study, wisdom, understanding, and all things creative.
This website (selling a statue of Saraswati) had really beautiful language to describe her:
She is the river of consciousness that enlivens creation; she is the dawn-goddess whose rays dispel the darkness of ignorance. Without her there is only chaos and confusion. To realize her one must go beyond the pleasures of the senses and rejoice in the serenity of the spirit.
The Gayatri mantra is said to be an incarnation of Saraswati and it is a prayer that is sung first thing in the morning by many practitioners.