So we start out with a total t. The first son gets $100 and 10% of what's left. So that would be: son = 100 + .1(t-100), with t = t - son.
The second son now gets $200 and 10% of the remaining total. Set that up as: son = 200 + .1(t-200). Again, we subtract t = t - son, to get the remaining total.
I'll let you set up son. After that, you set the equations for the three sons equal to each other, and solve. I set up the equations a little more programatically, adjusting a single variable. So remember when you set up subsequent equations after son to adjust for the change in the total.