how can i create third layer of video?

I take it that you want three inserted Pictures in Picture to play simultaneously instead of consecutively. Unfortunately, that cannot be done in one operation. Added layers of Pictures in Pictures will play consecutively but not at the same time. But there is a workaround that will accomplish it.

It is easiest to create a new project for the specific purpose of adding layers, then export the final product and import it into your original project.

So, create the first Picture in Picture overlay. Now share out (export) the project. Then import the movie back into your project, and add the second Picture in Picture layer. Repeat the process of sharing out the project, importing back in, and then the third layer. Now they will play simultaneously. Finally, export your specific purpose project and import the exported movie back into your original project.

-- Rich