Originally Posted by MarshmallowBlue
Well just to be clear, like Yooper said, don't rack until it's finished. Then stabilize in the new vessel.
As far as Head-space. Once fermentation is done, you can fill carboys pretty much right to the top as they won't be producing foam anymore. So three 1 gallon carboys filled up to the necks, minus the less slurry at the bottom of primary, and I doubt you'd be losing much mead at all (or you can store the leftovers in something even smaller like a growler or half growler).
Since the mead may very well need to be racked again, especially after adding more fermentables, another carboy (or three as marshmellowblue suggested) is really important to have.
It's important to reduce headspace, once active fermentation ends, so something smaller is needed in the long term.
You could use a three gallon carboy, and a 1 gallon carboy, for example.