Pro tip, well not really since I'm no pro. But what I've noticed is SuperSU hangs when trying to disable Knox if it's never been used to grant an APP super user permissions. It's like it's looking for the log to have something in it. If I run Root Checker, Titanium backup, or any other app that needs root permissions first, then SuperSU can disable Knox on the first try. Otherwise it hangs. I usually just use one of the Root Checker apps from the playstore.
As far as recovery, I'm using TWRP Official 2.8.7.0. Philz has internal storage issues if I restore a stock backup (custom roms work fine, but I like modifying my own rom.) I have no issues at all with TWRP with the latest version. A few versions back had issues with mounting external storage but no problems what so ever with TWRP lately.
My S3 is the SPH-L710 (Sprint version) on Boost mobile.
Good luck.