I'm making a homebrew Dragonborn variant with the following text:

Your body is covered in tough dragon scales. When you aren’t wearing armor, your AC is 10 + your Strength and Constitution modifier. Feral Dragonborn can not and will not wear armor and doing so will not affect your AC. It is foreign and uncomfortable to them. Feral Dragonborn also do not use shields and holding one will not affect your AC. If you have or later gain access to an Unarmored Defense feature, this trait overrides that feature and becomes enhanced. In this case, your AC is 10 + your Strength, Constitution, and Dexterity modifier.

I'm wondering how I should go about implementing this. I looked at the Lizardfolk Natural Armor for reference but it didn't really seem like quite what I'm looking for.

Thanks for any help in advance

You can do the first part (10+Con+Str), but you cannot do the second part that also adds Dex.

Ok but how? I’m trying to figure it out and I can’t see (at least on mobile) how to add a secondary attribute. 

as for removing the dex bonus you can use the Ignore modifier. 

Quote from Elocnodnarb >>

Ok but how? I’m trying to figure it out and I can’t see (at least on mobile) how to add a secondary attribute. 

as for removing the dex bonus you can use the Ignore modifier. 

 
Sorry, I would have responded sooner but I only just saw your post. My apologies. In answer to your question, you would add the two Abilities as separate Modifiers. I gave a more detailed explanation in the Homebrew Subclasses & Races FAQ #9: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).