In case of , CH3Cl, the 3 H atoms and Cl atoms sums up and give it a large dipole moment.
In case of CH2Cl2, the 2 H atoms add up to Cl atoms but one Cl atom cancels out half of the effect of other Cl atom and give it a smaller dipole moment compared to CH3Cl..
In case of CH3Cl, one H atom adds up and 3 Cl atoms cancel each others effect and give it a much smaller dipole moment...
