What are my chances of getting an RE-3 waiver (due to flat feet)?

Well, I guess it just depends on the reasoning of your discharge. I recently received a waiver because I also had an RE 3 discharge in 2014. I screwed around in AIT, and ended up getting discharged. After a while of being back home, I just felt horrible that I didn't honor my word and fulfill my contract. I got with a National Guard recruiter in July of this year. We went over my paper work, and he submitted my paperwork for a suitability review, and I got approved. Then the TAG General here in my home state signed my waiver and it was sent to the NGB and they sent me to MEPS for a physical, and two days later I was accepted back in. This whole process took about three months. You just have to be pretty passionate about your efforts of getting back in, and find a recruiter who is willing to work with you. Good luck!

There's no way to quantify a "chance." You can't get a 10%, 50%, 90%, whatever.

It's a shot in the dark, as the approving doctors quite literally have complete authority.

All you can do is have your Recruiter submit it and hope for the best.

A Recruiter can find out what the requirements are to be approved. It's on a case by case basis like PV2 Zehner said.