The topic title is confusing. What is pictured would be called a triple tap or something similar. It is not a three prong adapter in the usual sense.
I know that some will disagree-- but since light strings by the millions are plugged into cords this time of the year and will continue to be done for the forseeable future, my suggestion is to wrap it with non-combustible material to keep it dry -- then enjoy the lights.

The trouble with trying to seal it is that it probably will still let some moisture in. Any barrier that you set up will probably let water in faster than water vapor can get out so you have to "seal" it up in wet weather and open it up when dry to really do anything constructive. I'd either leave it be, maybe protect from direct rain or put on some dielectric grease.