What is a good AC in the underfoot era for tank?

Which bard songs? Are you using the ranger Cloak of Feather line? Tunare belt clicky? As others indicated, most recent Form of Defense/Ward clickies? Are you using a 1hnder/shield or dual wield? They're using a shield.

cleric buffs (sym+ac+HP vs Ageo line) + ranger AC buff (like 80-100? AC depending on rank) + a few AC clickies they have perhaps = 500 more?

A few improvements to upgraded augs / or if you are using a shield (which i don't think you said how you are setup) + focusing on AC buffs vs HP, could add up to that number.

who asks for help and then gets mad when people try to help? sounds like you're gear sucks and you just want to tell everyone that they are lying if they have better AC than you.

The biggest thing that it could easily be is they have popped a glyph and are saying those numbers which is bigger than basically anything else and account for multiple hundreds of ac.

Ranger Buffs / Cleric Buffs / Shield / Augs / *****SELF CLICKIE BUFFS****

On Aradune, I was in full raid crafted in Underfoot and 30+ AC augs in all slots and I could only breach 2800/2850 AC shielded and unbuffed. I'm almost 100% certain the raid-crafted gear (fully augged) has a bit more AC than the Rhodium set as well.

The only way I'd get close to 3K is with full buffs, Bard, and tribute. In terms of self-buffs that would be Champion's Aura (90) + Bracing Defense 3 (603) + Field Armorer 3 (432). Having those would likely push my numbers closer or just over 3,000. Also note that Bracing Defense means you wouldn't be running Final Stand.

You should request a screenshot for proof or even ask them to '/output inventory' so we can see the actual gear.