Any idea how to do add Transition effect with Masking between 2 video

Any idea how to do add Transition effect between 2 video.

for example i have 2 video & 3 video is black-White with Transition effect. now i want to add 3 video between 2 video as white portion show 2 video & black portion  show 1 video.

it is looks like animation. 
if any idea how can we do that please suggest

Thank you in advance.