Question
Is there any temple where deities are Brahma, Vishnu and Maheshwara?
Answer
Yes, there are a few.
Temples of historical significance
The very famous Prambanan temple at Java, Indonesia. This very old temple (and UNESCO world heritage site) from 9th century is dedicated to the trimurti. There are three temples, one for each Brahma, Vishnu and Maheshwara, there. Also, there are separate temples for their vahanas as well.
The main deity at the Elephanta Caves, Mumbai, India, (which is also a UNESCO world heritage site), is a large stone sculpture of trimurti.
The three heads represent three essential aspects of Shiva: creation, protection, and destruction. The right half-face (west face) shows him holding a lotus bud, depicting the promise of life and creativity. This face is symbolism for Brahma, the creator or Uma or Vamadeva, the feminine side of Shiva and creator. The left half-face (east face) is that of a moustached young man. This is Shiva as the terrifying Aghora or Bhairava, the chaos creator and destroyer. This is also known as Rudra-Shiva, the Destroyer. The central face, benign and meditative Tatpurusha, resembles the preserver Vishnu.
Temples where worship is still being carried out today
The Thripaya Trimurti Temple at Thrissur, India is another temple dedicated to trimurti. Here, the three deities are worshipped at once, as they are all placed on the same pedestal.
Thripaya Trimurti Temple is a Hindu temple in Irinjalakuda, Thrissur in Kerala, India. It is the only temple in the world where the Trimurti sit in one pedestal in one sanctum sanctorum.
As @NogShine mentioned in chat, the Mithrananthapuram Trimurti Temple at Thiruvananthapuram, Kerala is also another place where the trimurti are worshipped together.
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