Accessing mcs by _x prop value

I have 12 instances of a movieclip. These are arranged on the stage in three columns.
I would like to populate three different arrays, one array per column, with mcs from each column. In other words "all mcs._x == 60 go to array 1, all mcs._x == 130 go to array 2, etc.

I anybody could lend a hand would be greatly appreciated.