The radius of first Bohr’s orbit for hydrogen is 0.53Å . The radius of third Bohr’s orbit would be

The general formula of radius of an orbit is

${\mathrm{r}}_{\mathrm{n}}=\frac{0.53\times {\mathrm{n}}^2\mathrm{\AA }}{\mathrm{Z}}$

Here, n is Bohr orbit number and Z is atomic number. 

For hydrogen atom, Z =1 and the third orbit n = 3 the radius is

${\mathrm{r}}_3=0.53\times {\left(3\right)}^2\mathrm{\AA }$

$\therefore {\mathrm{r}}_3=4.77 \mathrm{\AA }$