How is it possible to have 3 moles of a gas in 1 litre if we say 1 LITRE of gas contains 22.4 moles ..Is it due to pressure or what?

Dear Student,

1 mole of gas occupies 22.4 L of volume only at STP i.e. standard Temperature (273 K) and Pressure ( 1 atm). 
3 moles of gas can occupy 1 L volume depending upon temperature and pressure. Greater the pressure, greater will be the number of moles and less will be the volume. This is because, 
$$\begin{gathered} \mathrm{P} \mathrm{\alpha } \mathrm{n} \\ \mathrm{and}, \mathrm{P} \mathrm{\alpha }\frac{1}{\mathrm{V}} \end{gathered}$$