I don't offset the divider/braces. I make each driver section 13"x13" on the inside of the cube. The top, bottom and sides are 1-1/2" thick, braces are 3/4" thick, and the baffle is 1" thick. The cabinet is 43-1/2" tall, 16" wide, and 14" deep. The bottom is 1-1/2" thick, then a 13" driver opening, a 3/4" brace on both sides of the baffle, second 13" driver opening, a 3/4" brace on both sides of the baffle, a third 13" driver opening, then 1-1/2" thick top.
Offsetting the 3/4" dividers allows the cabinet to be a little shorter but that is not relevant for triple subs since nothing will be sitting on top of the sub and having the dividers symmetrical makes the cabinet easier to build.