I know how to create homebrew items to add additional damage to offensive magic, but, I can't figure out how to add additional damage to specific kinds of magic damage. I want to make a 'Ring of Fire' in which it boosts fire based magic damage by +3, but doesn't for any other kind of magic. Is there a way to do that in the Homebrew items? I mean I know I can just spell it out in the details, I guess I'm specifically hoping for the mechanics to translate into the roll abilities in D&D Beyond (where it calculates the damage automatically when you use the roll feature in the app/website).

I'm pretty sure Homebrew tools here can't handle that level of granular tinkering. You're best of just adding it to the item notes. 

Metamongoose Is correct, the system cannot do that.