Circles $C_{1}, C_{2}$, and $C_{3}$, with centers $O_{1}, O_{2}$, and $O_{3}$, and radii $r_{1}, r_{2}$, and $r_{3}$, respectively, touch each other as shown in the following figure. Given $r_{1}=2 \mathrm{~cm}$, $r_{2}=1 \mathrm{~cm}$ and the angle $\angle O_{1} O_{3} O_{2}$ is $90^{\circ}, r_{3}=$ $\_\_\_\_$ $\mathrm{cm}$.
- $\dfrac{1}{2}(-3+\sqrt{17})$
- $\dfrac{1}{2}(3+\sqrt{17})$
- $\dfrac{1}{2}(-2+\sqrt{17})$
- $\dfrac{1}{2}(-3+2 \sqrt{17})$
