One after the other forming a line (2,1,3)
I believe the answer is:
in a row
I believe this is a double definition.
'one after the other' is the first definition.
(I've seen this in another clue)
'forming a line' is the second definition.
(I've seen this before)
(Other definitions for in a row that I've seen before include "Consecutively" , "Like all the pretty maids" , "successively" , "becoming involved in disturbance" , "Paul --, Swiss artist" .)