Calculating port length

So, I've never really been a big fan of ported boxes, mostly because I was always worried that if I didn't nail the specs to the T then it would sound terrible and blow the sub.  However, I am redoing my company's demo car and the main consensus is go ported for the 3 Rockford T1s that I'm puttin in there.

I have seen pictures of enclosures made for multiple subs that have just one large port.  This just can't be right.  It just doesn't seem like you could get each sub to perform the same when they are sharing one big port.  The other thing is that these boxes are typically partially fiberglass so the exact volume going into it is not known, can this blow a sub?  From what I understand, port volume and enclosure volume directly affect the tuning frequency of the box, which if unmatched with the manufacturers specs, can damage the sub.

Can somebody please help me figure out how I can build one slot load box with a common chamber for 3 subs?  Or should I just do 3 seperate boxes with 3 seperate ports?