The is no 3.0 firmware for iPhone 4, youre referring to your current baseband.
I'm pretty sure if you upgraded you're on 4.2.1 and no you can't downgrade unless you have saved SHSH Blobs. Even if you could downgrade the firmware, your baseband cant be downgraded to an unlockable baseband so you have to wait for an unlock.
The bottom line is that there is no unlock for iPhone 4 on 03.10.01 Baseband