Referring to previous work “by the authors”
I would avoid "the authors" unless it is exactly the same set of authors (preferably in the same order).

I don't see anything wrong with "B1, A2 and B3 have shown that..." even when A2 is one of the authors of the present paper.  It seems a little strange at first, but is quite common.  I have also seen phrases like "B1 and the second author have shown that..." to refer to a paper by B1 and A2 while avoiding mentioning A2 by name, but in more complicated examples it gets pretty unwieldy.