Ok, I'll give you a real answer, at least to the best of my somewhat dated knowledge.
ART: Air Reserve Technician in the Reserves, or just Technician in the Guard. Salaried GS-XX (with comp time, etc) for the bulk of their paycheck; part-time military member for the other part of their paycheck. Es are usually GS-6-9; Os are usually 12-13, although I've lost track with the latest and greatest attempts to attract/retain people. Typically fulfills the basic squadron functions from ARMS to Airfield Ops to DOV to DOT. My last OG was a Technician. They are civilians in uniform M-F 0730-1630, not counting gym time, lunch time, etc. 😉 They transform into military personnel (E-3, O-4, etc) after hours if they fly that night, sit SOF, whatever. In that status, they get paid the same as the part-timers: 1x 4hr block is a period. Their status on any given day is way more complex than I have time for here. Sometimes they can fly in civilian status, sometimes they can't. Sometimes they take mil leave, sometimes they take leave without pay, sometimes they're flying after their ART day ends. Doesn't matter unless you're filling out the 781 or you're one of them. Can't get Tricare unlike every other part-time military service member, has an expensive federal pension plan, and can legally work some crazy long days. Typically must be a part-time military member to hold the ART job, although there are a few Technician only people floating around. They can work until 50-something as a GS, then they can retire and end up with a federal FERS pension and their DOD Guard/Reserve pension.
AGR: Full time, salaried, military member. They're never a GS-anything, always a 1 on the 781. They get Tricare. They accrue years just like the active dude. They can PCS and go to school in res, but that varies between Guard/Reserve and I'm sure local norms. Certainly not the same PCS lifestyle as AD. My last ops squadron had ZERO AGR pilots. WG/CC had given all the AGR spots to MSG/MDG/etc. I've heard FWs have more AGRs for their alert missions. AGRs count against Congressional end strength, which is why it's not as easy as it should be to convert all ARTs to AGRs. They can retire at 20 years with check-of-the-month and Tricare just like the AD dude. They have a binding ADSC-like contract that historically has been easy to curtail. WOM is that might be getting harder as there are fewer guys lined up to take that AGR position. Administratively very similar to AD, but culturally as different as the unit they're in.
AD: What can I say? Salaried, Tricare, will PCS, will deploy a lot, will probably try to go to Guard or Reserve at the end of their ADSC.
All 3 are hemorrhaging people. None of the 3 can compete on salary or time off work with the airlines. Obviously if you want to live in NOLA and go fast, LAANG is a better fit than commuting to the MD-80 in DFW.