How to make a rounded corner triangle in XAML

I think the Path below will solve your problem. Only you need to change the points of your triangle according to your requirement. For more information about Path

 <Path Data="M 300 100 L 500 400 100 400 Z"
            StrokeThickness="48"
            StrokeLineJoin="Round"
            Stroke="Blue"
            Fill="Blue" />