Sorry for taking up so much of your time!

No worries. I’m happy to help.

I tried to do that for my own homebrew, and for a test, I took off all my armour. And weirdly, instead of giving me CON Score+3 as set, it gave me "unarmored (10)+DEX+CON Modifier + 3". What am I doing wrong? I definitely set it up as "Modifier: Set->Subtype: Unarmored Armor Class->Fixed Value: 3".

To create a natural armor AC of 13+Con mod requires a combination of 2 modifiers:

• Modifier: Set->Subtype: Unarmored Armor Class->Ability Score: Con->Fixed Value: 3
• Modifier: Ignore->Subtype: Unarmored Dex AC Modifier

For additional info, please see the Homebrew Subclasses & Species FAQ #9: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). And please check out the rest of the FAQs for even more helpful information to give you advantage on your homebrewing.

Quote from IamSposta >>

To create a natural armor AC of 13+Con mod requires a combination of 2 modifiers:

• Modifier: Set->Subtype: Unarmored Armor Class->Ability Score: Con->Fixed Value: 3
• Modifier: Ignore->Subtype: Unarmored Dex AC Modifier

For additional info, please see the Homebrew Subclasses & Species FAQ #9: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). And please check out the rest of the FAQs for even more helpful information to give you advantage on your homebrewing.

Oh, Ahh, yeah, no. That’s not a thing the system can do.

Quote from IamSposta >>

Oh, Ahh, yeah, no. That’s not a thing the system can do.

 That's a shame. So I would have to put a note on the feat that the AC must be calculated manually?

Yeah.

Out of curiosity, I am trying to homebrew a race with the same Unarmored Defence as the Barbarian. (10+DEX+CON) How would I do that?

EDIT: never mind! I somehow figured it out

Quote from OrsikBattlehammerII >>

Out of curiosity, I am trying to homebrew a race with the same Unarmored Defence as the Barbarian. (10+DEX+CON) How would I do that?

 

EDIT: never mind! I somehow figured it out

 
For anyone looking for this answer in future, it can be found in the Homebrew Subclasses & Species FAQ #9: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).