After proving the story didn't have to make complete sense as long as the movie was entertaining, the idea of a 3rd Friday the 13th movie wasn't out of the question. However, (blog post) it needed something special to (blog) make it worth filming... so it was made in 3D! That's all. There wasn't much else action movies they thought up before filming started, so prepare to have your eyes gouged out.
Source: Friday the 13th Part III Movie Review - Decker Shado