I am not able to offer a for sure solution because I dont have enough information and can only seem to find small pics of this product on the internet so I am not able to view it well. So I will offer what I can to help.
Perhaps you are not using a 4 channel amp? There also may be a problem with the output terminals on the box itself..maybe the wires inside coming from the subs are not connected properly to the outputs. How are you testing each sub individually? If you are testing with speaker wires coming from you amp well then we know that you amp is working...if you have taken apart the box and messed with the subs to test them maybe there was a small mistake made when reconnecting the wires? Like a positive connected to a negative or something? Maybe a fuse blew on your amp if you have one, or in your car? I cant be certain because there is not to much info to go on here so I hope that what I have given has helped you in some way...if each sub is working when you test them then it is nothing on or with the box except the wires maybe being reversed on accident or a bad connection on the output terminals on the back of the box, make sure those are connected properly on the inside. Other then that its probly not the box...maybe your amp is not big enough to handle all 3 subs and you are running a 2 channel amp and blew a fuse? You may or should get a 4 channel amp if you dont have one...like I said not to much info provided so good luck to you and hope I helped in at least some way!! :)