What is a three pronged fork called?

A fork with three tines ("prongs") is called, not surprisingly, a fork. The number of tines doesn't make a difference in what it is called, except that shape and/or placement might dictate that it is a "salad" or "dinner" or "seafood" fork. A larger fork with three tines might be a kind of serving fork. It's notcalled a "trident" because the trident is a type of three-tipped spear, and not a fork at all.