Well, the official list of URI schemes is here: https://www.iana.org/assignments/uri-schemes/uri-schemes.xht... (and another "unofficial" one here: https://en.wikipedia.org/wiki/List_of_URI_schemes#Unofficial... )

I think if you don't insist on them making sense as an acronym, there are still some interesting 3-letter combinations free (if you avoid the obvious minefields, that is).

E.g., I've never heard of qqq:// . There is a QQQ Trust apparently, but no network protocol AFAIK.