Three letters on the .org. That’s a category of name that only gets scarcer, because nobody is making more of them and the holders who have them tend not to sell.
ADT reads as an established acronym to almost everyone who sees it, which is the quiet advantage of a short letter string with existing currency. It also serves plenty of other readings: assistive device technology, adaptive design, applied data theory, any association or standards group whose initials land here. For a membership organisation or a technical body, three letters plus .org is close to the default shape of an institutional web address.
The practical case is simpler than the branding case. Short names survive being read aloud, typed on a phone keyboard, printed on a badge, and squeezed into an email signature. Every character you remove from a domain is friction removed from everything the organisation ever prints.
Three-letter .org names trade rarely and quietly. If it fits your initials, it’s worth a conversation.