That's right

You don't need to renew if your H1-B is valid for 3 years.

RIR is a different question. It stands for Reduction in recruitment. For you to use RIR, your school needs to be able to demonstrate that it is constantly looking for people to fill the position you are in, mostly through continuous advertising. At least it has to be advertising for a continuous 6 months prior to your filing RIR, and still looking. (I don't recall the specific time, but should be close.) It does not have anything to do with the length of your H1-B.