Trimurti

Skt., having three forms

Collective name for the three gods Brahma, Shiva, and Vishnu; the famous all-male trinity of the Puranas. When depicted, the three heads of these deities are shown on one neck, or even three faces on only one head - symbolizing that in fact, they are only three aspects of what is often regarded as the supreme absolute: the Brahman of the Brahmanas). As such, they represent the forces of creation (Brahma), preservation/maintenance (Vishnu), and destruction (Shiva).

Anyway, that's the original theory. In reality, Shiva devotees often see each of these three as aspects of greater Shiva; and other schools have again other interpretations.