I found this on gamefaqs for 5-3.
The triti follows two basic rules for its regeneration.
The first, and most important, rule is this. Whenever you remove any piece of the triangular membrane, ANY, repeat, ANY remaining piece with two thorns on an open edge will replicate a new triangular piece onto that edge.
So (taking the example of 5-3, with its two separate starting batches of triti), even if you've pulled all of the thorns out of the right batch of triti, the moment you use the forceps to pull off one of the pieces from the membrane, every triangular piece with two thorns opening onto an edge in the left batch will replicate a new piece.
How do you cope with this, I hear you ask? By removing all the thorns on the edges, of course. A daunting task, thanks to the thorns' constant regeneration, but that's what Healing Touch is for. (A note, be very careful when you're drawing your HT star. As a lot of people have already pointed out, higher star quality = more surgical bullet time). So, go into HT, remove all of the edge thorns, and start cutting, removing more thorns as needed, making sure you never leave an edge piece with two thorns facing the edge.
The second rule is a little screw-you from Atlus to us that can ruin the last stage of these operations. If a piece of triangular membrane is left isolated (edges not touching any other edges) and it has at least one thorn, it will replicate onto all three of its sides (especially painful if it's the very last piece and it suddenly grows a new thorn). The only way to deal with this is to pull thorns as fast as humanly possible (HT is a huge help here, but if you're at the end of the operation it's probably gone). If you do get caught by this, don't panic. If you move quickly enough, four pieces aren't that hard to deal with as long as you keep rule one in mind.