Why are there three forms of Artemis?

Because the Romans and the Renaissance confused matters, Artemis was identified with Selene, the Titan Goddess of the Moon because she was a Huntress and Apollo was confused with HeliOS - the Titan God of the Sun; and brother of Selene almost as soon as the Romans got a hold of the myths.

Artemis, being a virgin goddess, was also confused with Hecate through Persephone who was the Queen of the Underworld and another daughter of Zeus.