Is there an actually ruling anywhere about tarn mison in that situatoin?
The ruling is under the ACQUIRE A TARGET LOCK rules in you're rule book.
I'm not sure when you stopping reading my post
but here it is again:
EVERYTIME you ACQUIRE A TARGET LOCK you must measure to see if it's in range 3.
Most of the time an abilities like Tarn or FCS are in situations that have already be measured. Most ships can only shoot range 1-3, so if you can shoot you can pick up a target lock.