the density of 3 molar solution of sodium chloride is 1.25gm/ml.calculate the molality of the solution

Answer 1

3M NaCl = 3moles/L soln.
3 moles = 3*58.5 g NaCl = about 180 mL but that's just approximately correct. You need to have a better answer than that.
Then 1 L of the soln has a mass of 1.25 g/mL x 1000 mL = 1250 g. How much of that is water. It is 1250 g -180 g NaCl = about 1070 g H2O.
Then m = moles/kg solvent.