Three circles, with radii measuring 1, 2, and $3 \mathrm{~cm}$, are pairwise externally tangent, as shown in the given figure. Determine the radius of the circle that is externally tangent to the three circles.

![](https://cdn.mathpix.com/cropped/2024_05_01_30c9a294a58a6e4b8190g-035.jpg?height=374&width=374&top_left_y=2303&top_left_x=1503)