One needs to be careful in making sure that you are using the word “trinity” to describe the same conception of deity when comparing religions. There are many religions that have triads of Gods or Goddesses, and those that have Gods or Goddesses with triple aspects, but those are not identical to the Christian teaching of the Trinity.
For instance:
hinduism.about.com/library/weekly/aa061703a.htm
“We have to understand that basically, Brahma, Vishnu and Shiva are not three distinct deities, independent from each other, but they represent in fact the same Supreme Force, in its three different aspects.”
This is quite different from the Christian teaching about the Christian Trinity, which teaches that God the Father, God the Son and God the Spirit are, in fact, three separate individuals, though linked in a (to my mind extremely convoluted) specific way.