3) There are three circles that are externally tangent to each other in pairs. They have radii equal to 1, 2, and 3, respectively. The radius of the circumcircle of the triangle that has the centers of the three circles as its vertices is then equal to $\begin{array}{lllll}\text { (A) } 2 & \text { (B) } 2.5 & \text { (C) } 3 & \text { (D) } \pi & \text { (E) it is not possible to determine. }\end{array}$
3) The answer is $\mathbf{( B )}$.

By joining the centers of externally tangent circles, the segment obtained has a length equal to the sum of the radii. The triangle that has the centers of the given circles as vertices therefore has sides of length 3, 4, 5, and is thus a right triangle. Remembering that every right triangle is inscribed in a circle having the hypotenuse as its diameter, we have that the required radius is $5 / 2=2.5$.