This sounds like it should be pretty straight forward (through the method you’re attempting).
Have the smaller and larger rectangles set up and the gaze functionality with the ability to trigger the smaller rectangles.
Inside the level BP, first determine whether a smaller rectangle has been activated. Set this up for all of them. I would use an event tick and a sequence to always be checking each condition.
Event Tick > Sequence:
(1) Is A triggered? > Yes > Play Video (assigned to large rectangle #1)
(2) Is B triggered? > Yes > Play Video (assigned to large rectangle #2)
(3) Is C triggered? > Yes > Play Video (assigned to large rectangle #3)