The coordinate of the centre of mass of a system as shown in figure :-

 

$m_1=M (solid sphere)$

$r_{x_1}=0 r_{y_1}=a$

Hollow sphere

$m_2=M$

${\mathrm{r}}_{{\mathrm{x}}_2}=0{\mathrm{r}}_{{\mathrm{y}}_{2_{ }}}=0$

Disc.   $m_3=M$

$\begin{array}{r}{r}_{y_3}=0 \\ r_{x_3}=a \\ r_{x_{\mathrm{cm}}}=\frac{m_1{r}_{x_1}+m_2{r}_{x_2}+m_3{r}_{x_3}}{m_1+m_2+m_3}=\frac{0+0+aM}{3M}=\frac{a}{3}\\ r_{y_{cm}}=\frac{m_1{r}_{y_1}+m_2{r}_{y_2}+m_3{r}_{y_3}}{m_1+m_2+m_3}=\frac{Ma+0+0}{3M}=\frac{a}{3}\end{array}$

So the coordinate of mass if system will be $\left(\frac{a}{3},\frac{a}{3}\right)$