To synchronise these videos, press play and pause immediately on each one. Set them to the beginning again.
As a suggestion, start with the top video, and as you are counted in, (1,2,3 2,2,3) bring in the video on the right, and then the bottom.
Finally, start off the video on the left after 8 bars (count yourself in 1,2,3 2,2,3 3,2,3 4,2,3…).
Allowing for perhaps a slight delay to the start of the video, being the nature of the internet, it should all sync up well.