How to calculate the force inside a column being pressurised by a pump. Do we have to include the length of the column in the calculation. Say the column was 3 metres tall the pressure was 3000 psi?

no the column length does not matter. All that matters is the pressure inside it (I'm assuming it is a hollow column with a piston inside), and the cross sectional area inside the column, equal to the surface area of the piston. If you're talking about a hollow tank shaped like a cylinder and a pump is pressuring it up, then the length may matter. What "force" are you talking about?