As you have done begin by calculating the equivalent capacitance of C1, C2, C4, and C5.
Ceq = 40nF
You know that the total voltage across C3 and Ceq must be 48V. This is because C3 and Ceq will have the same charging profile, and thusly the same amount of charge on both capacitors.
Qeq = Q3
and given that:
Q = Vcap * C
C3 * V3 = Ceq * Veq
V3 = 1.67 * Veq
Remembering:
48 + Veq + V3 = 0
we get:
48 + Veq + 1.67 Veq = 0
thus VeqV3 = 30.1V and V3Veq = 17.9V.
From here, rinse and repeat for the remaining capacitors.