When I cut it into 3 individual strips, the side channel wouldn't stay up in the frame when I rolled the window down. When I only snipped the sides of the channel (so I could bend it 90*) and kept it all in one piece, it stayed up fine. It's also important to overlap the corner flaps in the right way or the window will bind the upper channel as it pushes up that last 1/2".
I'll see if I can get a picture of what I mean.