Flashed a new firmware, now wifi won't turn on
Based on our chat, you did not do a "clean flash" ( Factory reset + wipe Dalvik cache + wipe, preferably format /system) of stock ROM using Odin.

Clean flash is always recommended if you are switching between ROMs from different sources for same Android Version or switching between versions. This is to ensure that /system does not have remanants of the earlier ROM.

What you did while switching ROM from Slimkat to stock is a "dirty flash", (see Clean flash Vs dirty flash  which could have resulted in your Wifi problem.

Suggest you first clean install stock ROM and check WiFi connectivity. In case that doesn't work suggest you update modem to the latest version from complete modem list for S3 either flashing by Odin or custom recovery