cant give a sure answer for that, as I dont have any mp7 mags, but I used to be able to fit 3 mp5 mags into a normal mag pouch. I think they are similar size, might be worth a try!
I was looking for some when I got my TM MP7, I tried an MP5 triple pouch which I already had but didn't like it. I saw the BFG ones and was going to buy them but couldn't find them in CB anywhere at the time. I then found the below review for custom MP7 taco's on youtube, I bought some of these and they have been great, attached pics shows mine. These cost £150 for all 6 posted.
Shortly after buying them I saw that HSGI then released some extended tacos which fit mp7 mags, I think they are £25 each from Tactical kit, so slightly more expensive as you then have to pay delivery as well, but will probably be a quicker turnaround from order to receiving them.