Leviton Triplex unavailable/discontinued. Alternatives?
New box (either add, or replace smaller with larger), pair of duplex, 4 outlets. 

Sure, you want not to do that, but it works, and always has worked.

If you can handle it being 15 amp rather than 20 and a large pricetag, Legrand adorne makes what's claimed to be a 1-gang triplex pop-up outlet. Never seen one in person, and I'd wonder about long-term reliability with it having moving parts.