Brahma is the Lord of creation. Knowledge (’vidya’) is the basis for creation. So Saraswati is his consort and she is said to reside in the tongue of Brahma.
Lord Vishnu sustains the world; to support this work, the primary requisite is ‘Bhoomi’ (land) and ‘Sri’ ( all forms of wealth) and so they are the consorts of Maha Vishnu. For the Lord that preserves and maintains the Universe, all the living beings expect grace and compassion. Compassion springs from the heart. [We often call a rude person as heartless, don’t we?] Thus Vishnu has His consorts, verily in His heart.
Lord Shiva is the deity of dissolution, and Shakti or energy the key to destruction. So Shakti is the consort of Lord Shiva. As the energy is inseparable from the subject possessing it, Shakti is depicted as verily sharing Shiva’s form (’Ardhanareeshwara’).