I am confused by your diagram. I can't tell how many bases are in each BG. I can't tell which way they are facing. I can't tell where 'behind' is. And which BG does

belong to?
First of all,

could never contract in its own battle group and be placed behind another BG. It would be placed behind a base in its own BG.
If

and

are enemy BGs facing the same way or opposite directions, then the appropriate rule would be "Side Edge to Side Edge Only" on pages 76 -78. In that case you could turn 90 degrees to face the enemy and draw them directly into melee without going through the Impact Phase. Turns are allowed in Restricted Areas. Of course, then your flank is exposed to the

BG which will then become the

BG.
FWIW, page 75 says that a BG may move into overlap position against a BG and ignore the RA of all other BGs to do so. On the other hand, page 74 says the Restricted Area applies to 'normal movement only' (not defined). It says you can conform to an overlap position against another enemy BG as long as it ends its move at least partly in front of the BG whose RA it was in at the start of the move. If the conform takes it out of the RA of the 3rd BG , then the conform can't be done. I, personally, think these two pages contradict each other.