If the transitions are all the same but with different video, you just need to build one in a precomp and then duplicate it for each transition you need, then replace the video in the precomps. You can replace the video in the precomp by selecting the video layer to be replaced. Go to your project window and select the replacement layer and press(PC) cntl+alt+/
I made a sample project, where I made 2 transitions, duplicate the one of them, and use the replacement method, then apply that to your project.