Are you sure it's Buddha, Lyndsey? Some of the Hindu gods are shown with three heads: Lord Shiva for example, is sometimes depicted with three heads.
Or the Hindu 'three in one' Trimurti, Brahma, Shiva and Vishnu are also portrayed as having one body but three heads.
I also found this on the web:

I think I have seen pictures of a number of three faced Tibetan and Chinese statues of Bodhisattvas including Tara, Avaloketesvara and Vajrasattva. There are a number of sybollic meanings of the number three in Buddhism: The tree times of past, present and future; The three poisons of ignorance, anger and desire; The three states of indifference, happiness and sadness... and the three jewels of refuge, Buddha, Dharma and Sangha and there are probably more.

I think that the three faces normally would relate to the bodhisattva's ability to have overcome three obstacles such as the three poisons, the three times or the three states.

The way you are likely to find out which bodhisattva it is you have is to look at the hand positions (mudras) and number of arms and the ritual objects they they are holding. You can then look at wikipedia or some other source and look up different bodhisattvas and compare.

Norbu