Interception
 

B10 intercepts the football in team B’s end one and during the return fumbles the ball from the end zone ,and the ball goes out of bounds at the 3 yard line.
Where is it addressed in Rulebook? Touchback?

Which rule set?

In NCAA (and therefore TX HS), this is a touchback. A forward fumble out of bounds between the goal lines belongs to the fumbling team at the spot of the fumble. That is in the end zone, and Team A was responsible for it being in the end zone (the forward pass that was intercepted).

Yep. Processed exactly the same as the other thread.
Fumble forward and OOB -> return to spot of the fumble
That spot is in B EZ -> what was the impetus?
Forward pass by A intercepted by B -> Touchback