either in fillet tab (extra option for non-coplanar) or in blend tab with a corner selected (will act like fillet - thats why i'm more towards option in fillet tab) , both doing same thing, cut a distance and make blend together
the way i'm imagining cutting is a sphere growing radius from corner introduced by user, where sphere intersects with lines - make a cut & blend after, if growing sphere(radius) meets 3'd intersection point, algorithm fails
ofc if wont be fillet radius, but calculated radius (technically not part of fillet)