This is Vishnu accompanied by his consorts, Sri Devi and Bhu Devi. They are the Goddess of wealth and Goddess of Earth respectively. Vishnu is identified by his attributes. He is holding a lotus, discuss (square rather than the usual circular type), conch shell and mace (clockwise starting from his lower right hand).
The usual attribute of Sri Devi, one on Vishnu’s right, the breast band is missing. Another difference, as compared to Vishnu triads from other parts of south India, is the hand of the goddesses use to hold lotus. Here the lotus is held in the ‘outer hands’.
The pedestal is an integral part of the bronze and appears to have been cast a one piece. See photo below. The prabhavali, framing the triad, is broken. But Vishnu has his own prabhavali/halo and this is quite common among Kerala bronzes.
This bronze seems to have made using a combination of traditional South Indian lost wax method and wax thread method. The later one is commonly seen in Bastar bronzes.
Kerala bronzes are rare and it is rarer to find a near-complete set of Vishnu triad.
This bronze is about 15 cm in height. There is significant ritual wear on their faces and may date prior to the eighteenth century.
Note: B N Aryan and Abraham Papali are of the view this Vigraha could be anywhere between 14th-16th centuries.