Cleric780

i got 3 of the MA Audio MA120D6 12" the box specs for a vented box is 2cu/ft per sub and the square vent is 21 sq/in and 14 inches deep now i want to put all 3 in one opne chamber 1 port box i know the box should be 6 cu/ft but how bug the port 63 sq/in by 14 inch? all i did was tripple all the numbers..is that right or wrong? a round vent is (3)3''..what ever that meants but i want a square port though