The Crimson Binome
Hero
I was going with a level 17 adult red dragon, because ancient red dragons are level 24 and there's no such thing as a character of equal level. I guess you could go with an ancient brass or white dragon, which are level 20 and have AC 20, but sticking with level 17 means we get to avoid some of the silly capstone abilities.So if your talking 3 level 20 adventurers they should be fighting an ancient dragon with an AC 22 and a flight speed of 80ft (160ft if they dash away for a turn) that is nether low AC or slow...
A speed of 80 isn't very fast, compared to the range of eldritch blast (120), let alone a longbow (150). The dragon's breath only has a reach of 60, which means it can't hang out 125 feet away and then move within breath range and get out of blast range after that. If it attempts hit-and-run tactics, then it will have to spend at least one round where it's within blast range and can't fight back.
The CR is generally a good base line, in the context of fighting six of these things in a day without risking death. A single dragon is a chump compared to the immense resources and focus of a party.That said if your building your party out as strait killing machines and gods of war... then sure they are killing higher than there level. That's not a Dragon CR issue its a power game style and as such you will have to alter combat above CR to challenge them. The CR is a generally good base but it can't cover all parties.
What are their stats AC, to hit, HP, classes, level ? if you don't mind me asking.... just so I can see your comparison.
As for stats, just consider a tiefling fiend/tome warlock 17 with eldritch blast. Nothing fancy. It doesn't even have to be a tiefling, given fiendish resilience, but this is the character I saw in action. That's four attacks at +11 to hit against AC 19 for 1d10+1d6+5 damage, per round, every round, against the dragon's 31 fire damage every third round... unless the warlock saves, in which case he only takes 15.
That's not a "killing machine" or "god of war"; it's a level 17 warlock without any feats or (in this example) magic items. (Realistically, a level 17 is probably going to have some magic items, but let's ignore those for now.) That's not playing intelligently on the part of the PC. That's not using readied actions, or pushing the dragon out of range to spoil its breath. It's not using any of the high level spells available to the warlock - no Foresight to gain advantage on everything, or Forcecage to end the fight in an instant without requiring a save. That's just a dragon getting in over its head by messing with the wrong individual. (To contrast, that dragon would have a much better chance if it dropped into melee against this warlock. Even with only getting one legendary action, its damage output against AC 13 would eat through the warlock's ~170hp in about three rounds, unless the warlock did something about it.)