Trimurti is the worship of Brahma, Vishnu and Shiva together. Trimurti worship is found in many Shiva temples. Trimurti represents the form of three great powers acting jointly – creation, preservation and destruction.
Amasumadhdagama and Uttara Karanagama give elaborate descriptions of the forms of Trimurti. The ekapada Trimurti representing a pacific aspect, should be samabhanga (standing erect) with only one leg upon a padma peetha. The color of the deity is crystal clear, and he has three eyes and four arms. The ears may be adorned with kundala (earrings) set with pearls and the head, with matted locks (jata mukuta).
From the right and left of this erect figure of Shiva should be seen emerging those of Brahma and Vishnu respectively. In sculpture they ought to be shown above the waist. Brahma should have four faces and four arms. Similarly, Vishnu should have four arms, with the front two folded. The whole composite piece should have a prabha mandala (halo).
Another way in which Trimurti may be represented is with Brahma and Vishnu being sculpted from either side of a large Shivling.
Thirdly, they may be represented each one separately with all their characteristic, standing side by side on the three padma pithas.
Fourthly there may be three separate shrines in a temple with the middle shrine containing a Shivling and the right and left containing images of Brahma and Vishnu.
Finally, in a temple where there are three contiguous shrines the same as the above would occur. The central shrine may also be that of a Nataraja or other forms of Shiva.
Manasara states that the vigraha or murti of Trimurti may be made of gold, silver, copper, blackstone, wood, lime plaster or mud. They will be of three types – uttama (gold), madhyama (wood) and adhama (mud). These vigrahas can be moveable or fixed. Of the varieties of vigrahams, the full figure (citra) is considered utama. The bas-relief (arda chitra) is considered madhyama.
In all important Shiva Temples in South India, one finds these images, mostly carved on pillars. An excellent specimen is found in the temple at Tiruvottiyur. Another one is in the Shiva Temple at Jambukeswaram where, in addition to Shiva, Vishnu and Brahma, the figures of hamsa, garuda and nandi, the vehicles of the deities are seen.
There is a unique but little known temple of Trimurti in Mahabalipuram in Tamil Nadu. It was built by Narasimhavarman I Pallava (630 – 670 AD). This is a triple-celled temple, with each cell dedicated to Brahma, Vishnu and Shiva. Brahma in the shrine, rather unusually has only one face.
Source – Encyclopedia of Hinduism Volume V page 127 – 128 - IHRF