The centers of the three new circles form a triangle. The diameter of the new circles is $R-r$, so the side length of the triangle is $R-r$. Call the center of the concentric circle $O$, two vertices of the triangle $A$ and $B$, and $A B$ 's midpoint $D$. $O A$ is the average $R$ and $r$, namely $\frac{R+r}{2}$. Using the law of sines on triangle $D A O$, we get $\frac{\sin (30)}{A D}=\frac{\sin (90)}{A O} \Rightarrow R=3 r$, so $\frac{R}{r}=3$.