The two in parallel have a total of 60uF and the other is 30uF
If you connect across this combination the ratio of the pd across the parallel to the pd across the single will be 30 / 60 (=1/2) because V= Q/C (V is inversely prop to capacitance)
So it must be 3V across the parallel pair and 6V across the single. This gives the maximum 6V across the single capacitor.