According to Dalton's law of partial pressure: The pressure exerted by a mixture of gases is sum partial pressure of each gas exert on the container.
![P_(total)=p_(1)+p_(2)+p_(3).......](https://img.qammunity.org/2020/formulas/chemistry/high-school/sx7gc7xak46k6pkby2d971sl0sqew9lgz6.png)
Here we have mixture of three gases so the total pressure of three gases will be sum of their individual or partial pressures
![P_(total)=p_(1)+p_(2)+p_(3)=3345=988+1289+p_(3)](https://img.qammunity.org/2020/formulas/chemistry/high-school/glcbjiznwkwh3noyz5j0o7m4s7e442m2ik.png)
Therefore the partial pressure of third gas will be
![p_(3)=3345-(988+1289)=1068mmHg](https://img.qammunity.org/2020/formulas/chemistry/high-school/hi22oup52uv6viwsrm7uz9bto0umqdduak.png)
The pressure of gas#3 is 1068mmHg