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 \( r_1 = 2 \) cm, \( r_2 = 1 \) cm and the angle \( \angle O_1 O_3 O_2 \) is \( 90^{\circ} \), \( r_3 = \) _____ cm.
