C# Polygon Border Algorithm

I have a 3-sided triangle polygon. My task is to draw and re-calculate a border that's 250m away from the the 3 triangle vertices always always.

 

What will be the c# math algorithm to draw and recalculate the 250m border around the polygon. Keep in mind, if a user drags any vertex to a new location, the border has to recalculate to 250m from the new location.