Slitherine

If I have two BGs, side by side, they form a BattleLine. However, if I have a third BG positioned at the 'join' behind those two, so that it is half adj to one and half adj to the other, does that BG also now become a part of the battleline? Or do BGs have to be in full base contact with one or the other?

if it is touching it is part of the BL