You need to give them the same denominator so that would be 12.
3x4 = 12
2x4 = 8
4x3 = 12
1x4 = 4
So that would be 8/12 and 4/12
Then you would add the numerator so 8+4 = 12 so the mother and the son together get 12/12 of his property which is everything. So the daughter gets nothing.