Thoughts on fixing this. Shields complicate things, but if they must add to AC then:
Highest heavy AC + heavy shield should be the highest AC.
Highest heavy AC + no shield should be the same as the highest medium AC and light AC (I can't see a reason why not..)
Don't screw people in no armour too much.
Therefore:
Light armour should be 11-14 + dex
Medium armour should be 14-17 + dex/2
Heavy armour should be 16-19
The issue then is that if your dex isn't 20, you should take heavier armour. So I'll be bold and suggest that 18 is a more reasonable high dex (who knows really), giving:
Light: 11-14
Medium: 13-16
Heavy: 15-18
Or, I don't know, remove Dex from AC (it's already a save against Fireballs and such)?