3-socket replacement extension cord end?
This is one of the places where code and consumer parts don't match. I have tried in several cases to make code approved triplex adapters , even as a licensed electrician, I cannot do it legally , even with listed parts, I have asked several major mfg of cord caps for a way many years ago, because what you ask is reasonable, but in my state there is no way to do it because there are no listed "cord caps" with 3 outlets but it would make them $ if there were in my opinion. At this time I have not found a listed model to do what we both and many others want.