Scrypt is the realm of ASIC and has been for years - you need to reconfigure your miner software to use skein or groetsl (if either of those work on AMD hardware, you might need a different miner version).

 There is NO BLOODY WAY you were getting an actual 3+ Gh/s scrypt mining rate from any GPU, I'd estimate a R9 390X actual Scypt hashrate would be closer to 1 Mh/s (perhaps a little over).