Leave a crossroads and transfer from A2 to A3? (4,4,4)

'a3?' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'leave a crossroads and transfer from a2' is the wordplay.
I cannot really understand how this works, but
'a' could be 'one' and 'one' is present in the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'a2' could be 'a' and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

'to' is the link.

Can you help me to learn more?