Does a shield like this exist?

20 or more hp constant
-3 damage(%)

I've never seen one being sold, I always see them with a -2 in stance or -2 enchanted or something like that..

yes it can exist.

For the Received damage - modifiers, the max is 2 for while enchanted and in stance, and 3 for up to 10% chance or while hexed. So yes, you could have a 30 hp always -3 (10% chance) shield. It's just that the % chance isn't as desirable as the stance/enchanted mods, so you don't see it as often.

I see, I have always wanted a shield that is around 20hp and 3 8% or something like that, I've just never seen one around.