Question

If we are said to find pressure at sea level of 3000m,shall we consider the atmospheric pressure also

Answer 1

Hydrostatic pressure at any height below the water surface is calculated by P = h d g where is the height below the open water surface, is the density of water and is the acceleration due to gravity. So if you want to calculate gauge pressure at height then use formula P = h d g + P°. P° is the atmospheric pressure.