Hey Tony,
G0 is a motion command so first the carriages move to Z=0, X=123 and Y=321. If you then click on the zero button for these axes, or enter G92 Z0 X0 Y0 into the mdi, this coordinate is now the current offset, so Z=0, X=0, Y=0.
If you are at machine home and want to set the offset without moving there first, you can enter:
G92 Z0 X-123 Y-321
This will have the same effect as above, but your carriages will not have moved. It will simply set the new offset coordinate system, i.e. it will make that while you are still at machine home, this home position is now -123 mm on the left of your new zero coordinate and -321 mm towards front of the new zero coordinate). You are still at machine home, but the offset is valid the same way as above. So if you then enter:
G0 Z0 X0 Y0
the carriages will move to your new zero coordinate (which is at absolute machine coordinate Z=0 X=123 Y=321)