No the 3gb is from the opencl call to determine the size of your card. Could be related to a wrong gpu platform, or a driver installation gone wrong. You should do a driver uninstall with wipe and do a reinstall.
Well, if I stop the miner and run the --list-devices command, all the devices are read again with 8gb, so it is the "free memory" what you see from the tbminer --list-devices command, not the hardware memory