crimson dragon has 12 attack. so if it had adrenaline, wouldn't it deal 12 then 4? which isn't a multiple of 3
6 attack creatures are fine because the total damage is still a multiple of 3. 6+4+2=12
EDIT* and if we really wanted to get into technicalities. bonewall, thorn carapace, fire shield, fog, procrastination have a chance against an adrenalined 6 attack creature to not deal a multiple of 3.