Draw a perpendicular from the center of the
circle to the midpoint of a side of the triangle.
Now draw a line from the center to the vertex
of that side. You now have a right triangle with
a base angle of 30º, side adjacent to that angle
of 3, and hypotenuse = r, the radius of circle. So,
cos 30º = 3/r, r = 3.46, the diameter
is twice that, 6.92 round to 6.9