Elven Accuracy

Is there a way we could make 3d20 take the highest for this feat used by elves?

If you mean keep the highest roll from the three rolls, you can achieve that simply with Roll20's built-in dice roller.

/r 3d20kh1

The in-line version looks like this. [[3d20k1]]

I was thinking for the OGL 5e sheet, or within the macro for the attack roll

I have been using this and it works great.    In your Attributes, locate the one called rtype  and paste this in.   It will now ask if you are rolling Normal, Advantage (for non attacks), Elven Accuracy, or Disadvantage.  that way you dont have to modify all your weapons, just pick each roll what type it is.  When it rolls EA, it rolls 1d20, then rolls 2 more, keeping highest

{{query=1}} ?{Advantage?|Normal Roll,&#123&#123normal=1&#125&#125 &#123&#123r2=[[0d20|Advantage,&#123&#123advantage=1&#125&#125 &#123&#123r2=[[1d20|Elven Accuracy,&#123&#123advantage=1&#125&#125 &#123&#123r2=[[2d20k1|Disadvantage,&#123&#123disadvantage=1&#125&#125 &#123&#123r2=[[1d20}