Here's why:
* Hogsmeade is a restricted village. Third years are not allowed to go there without a signed permission slip from their parents or guardians.
* Harry doesn't have a guardian. He's an orphan, and his aunt and uncle are not going to sign a permission slip.
* He is allowed to go with Hermione. In the book, Hermione gets permission to go to Hogsmeade, and Harry sneaks out with her using the Marauder's Map. However, this is technically breaking the rules, and it's not something that's encouraged or condoned in the books.
So, while the movies show Harry going to Hogsmeade in Year 3, that's not accurate to the books. He doesn't actually get to go until Year 4.