allright one mole of MgCl2 is 94 grams (hence magnesium 24g and two times chlorine 70g).now if we were to make a 0.2 molar solution in one litre of solution we would add a fifth of a mole of MgCl2 which equates to roughly 18.8 grams of MgCl2 and 981.2g of water.To increase this to 3 liters we simply times it by three to get 56.4g of MgCl2 and 2943.6g of water.

does that answer your questionand good luck.