The Athlon II x3 455 is AMD's only CPU offering that is priced close the the G620 right now. The only other competitive products from AMD are the A4, which is a dual core APU and lower end A6 APUs, which are quad cores, but clocked lower than the A8 and with weaker integrated graphics.
As for unlocking the extra core, it is really the luck of the draw, there is no guarantee that the additional core is functional or will be stable. AMD usually locks the cores on their CPU for a reason, most of the time you probably have about a 30% chance of a successful unlock. If you do get the X3, do not count on being able to get that fourth core working.
That said, the Athlon II X3 might be the better all round CPU than the G620, it's clocked significantly higher, which makes up for AMD's inferior arch, and the third core would probably help in some of the more CPU demanding titles. The G620 is an okay gaming CPU, but its low clockspeed does limit it. Pair either CPU with a $65 or higher GPU and you will generally get better gaming performance than the A8 3870k on its own.