[Sage Advice][1] 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][2] is one way.

> Breastplate 14 + Dex modifier (max 2)
> Half Plate 15 + Dex modifier (max 2)	--	--
> Plate 18

[Normal armor][3] 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][4], 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.


  [1]: https://media.wizards.com/2020/dnd/downloads/SA-Compendium.pdf
  [2]: https://www.dndbeyond.com/magic-items/molten-bronze-skin
  [3]: https://www.dndbeyond.com/sources/basic-rules/equipment#ArmorandShields
  [4]: https://www.dndbeyond.com/classes/artificer#Armorer