3 concentric spherical conducting shells of charge 4q on the inner , -2q on the middle and -5q on the outer shell. The charge on the inner surface of the outer shell

There is only one capacitor in this set-up: one terminal is connected to the middle sphere, and the other terminal is connected to both the inner and outer sphere, which is why they are at the same potential. You can't view this as two separate capacitors in series/parallel because of the charge on the inner/middle spheres, for example, are not the same. You will need a more sophisticated analysis to continue in this vein, and it is not required here.
Assume a charge ofQ'on the inner sphere, and make use of spherical symmetry to find the potential difference between the inner/middle sphere and the middle/outer sphere using Gauss' law. Since the potential difference is the same, you can solve forQ'.