D3 is an inter-dimensional being that supervises 3-dimensional space in the universe.
Little is known about D3 other than that he serves Tokimi, one of the three Chousin. The other two Chousin, Washu and Tsunami also have authority over him as well.
It is not known to what extent D3 (or the other dimensional stewards) exercises free will, or if he is an automation. He did help raise Ran at Tokimi’s request as well as not tell him about his siblings, but he refuses to allow Ran to call him Dot despite being Tokimi’s child. Which is why Chibi-Ranma calls him Deezy. He doesn't appear to have a direct link with Tokimi, as he speaks with her using normal spoken language.
22 different Dimensional Stewards or "Ds" have been shown in total, with each ruling over an ascending Dimensional space. D4 has jurisdiction over 4-dimensional space, D5 rules over 5-dimensional space, and so onwards.