Go on the AOA opportunities and look under the "institution information" tab. It will tell you whether the program allows 3rd years to rotate or not. I would recommend that you choose a program that isn't going to be your no.1 choice, since you will probably not be a rockstar yet...
Also, I agree and disagree with the poster above. Yes, you won't know enough to be ready for EM as a 3rd year. But, when will you ever know enough? As a 4th year, I still don't feel like I know enough. As long as you are solid at getting an H&P in an efficient manner (<10 minutes) and study your emergent diagnoses, you will be fine. Attendings are usually happy to teach any student if you show that you want to learn.
Honestly, I felt like I didn't know anything going into EM midway through my 3rd year. Unfortunately, none of my procedure-heavy rotations (OBGYN, surgery) allowed students to do anything. So guess what, the ER is where I learned how to suture, do I&D's, pelvics, intubate, start IV's, etc... So it was a great experience for me and I'm glad I did it. When it came time for my required 4th year EM's and my audition, I was able to shine since I understood the general flow of emergency medicine and what is expected of me.
This is just my own opinion based on experience. Hope it helps