The radius of electron's second stationary orbit in Bohr's atom is R. The radius of the third orbit will be

3 R 2.25 R 9 R $\frac{R}{3}$

2.25 R

$r \propto n^2 \Rightarrow \frac{r_{(n=2)}}{r_{(n=3)}}=\frac{4}{9} \Rightarrow r_{(n=3)}=\frac{9}{4} R=2.25 R$