There are probably a few ways to do that. One would be to use shapes, add repeaters and animate the copies, and create multiple layers of that to make your triangle shape build. Then use that as a matte for your video.
You might all be able to do it with Plexus, but before you go buy that, you might want to talk to somebody who owns it and find out for sure. I don't have it, but in some of the videos it looks like they do similar types of things.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.