Sage Advice makes clear you can only use one formula for calculating your AC at a time.
These methods—along with any others that give you a formula for calculating your AC—are mutually exclusive; you can benefit from only one at a time. I
The Molten Bronze Armor is one way.
Breastplate 14 + Dex modifier (max 2) Half Plate 15 + Dex modifier (max 2) -- -- Plate 18
Normal armor is another, and Efreeti Chain +3 is Heavy Chain so it would use this formula:
Chain mail 16
So, Molten Bronze Armor Plate would be 18 and Efreeti Chain would be 16 + 3 = 19. You can choose to take the 18 or the 19, but Rules as Written, you can't do 18+3. And you can turn either into Arcane Armor, but not both:
As an action, you can turn a suit of armor you are wearing into Arcane Armor, provided you have smith’s tools in hand.