It appears that the Leviton Triplex outlets are no longer available and the product is listed as discontinued. My best guess is that they weren't technically code compliant?
They were a rather brilliant solution for fitting 3 ground NEMA compatible outlets into the space of a normal outlet.
Are there alternative solutions available?
Edit: The goal is to avoid messing with an outside wall. I have an existing weatherproof outlet where I need 3 plugs. Making it bigger is possible (and in my range of skills), but I'd rather avoid doing so.