Mūrtitrayam — A Context-Specific Triad of Forms
Mūrtitrayam literally means 'three deity forms.' The expression can be used when a particular temple, text, or ritual discussion groups three bimbas together, but the identity of those three must be learned from that context. It should not be presented as one invariant three-image scheme required in every Śrī Vaiṣṇava temple.
Temple Bera Systems
Vaiṣṇava Āgamic and temple traditions distinguish several functions of consecrated images, using terms such as the fixed dhruva or mūla-bimba, kautuka, snapana, utsava, and bali beras. A temple may not possess every category as a separate image, and local Tamil names and functions do not always map one-to-one onto a single universal list.
The mūlavar is the fixed presiding form in the sanctum. Some celebrated forms are svayam-vyakta, while many others were installed through proper pratiṣṭhā; self-manifestation is not the general definition of a mūlavar. The utsavar is a movable form used for processions and festival services. A kautuka or another smaller worship bimba has its own role according to the relevant Āgama and temple custom.
One Bhagavān, Distinct Services
Where several consecrated forms are present, Śrī Vaiṣṇava worship does not treat them as competing deities. Bhagavān graciously accepts different bimbas for different kainkaryams. The exact 'three' denoted by mūrtitrayam must therefore be identified rather than assumed.