Using the LCM method to find their combined efficiency, let the total work be the least common multiple of 8 and 12 days, which is 24 units. X completes 24 units in 8 days, so X's efficiency is 3 units per day. Y completes 24 units in 12 days, so Y's efficiency is 2 units per day. Since Z joined them and we are given they all finish the work in 3 days, Z's efficiency plus X and Y's efficiency equals 8 units per day, meaning Z's efficiency is 3 units per day. The ratio of efficiencies for X, Y, and Z is 3:2:3. Based on this 3/8 proportion of the total Rs. 400, X's share is 3/8 times 400, which equals Rs. 150.