The capacitance of a parallel plate capacitor is given by
C=εoεrAd,
where A is the area of the plate and d is the distance between them.
In the first case, there was no dielectric medium, i.e., εr=1.
So we have C=εoAd
In the second case, the distance is doubled, i.e., 2d and a substance of dielectric constant 3 is inserted between the plates.
Hence capacitance will change to
C′=3εoA2d
So, C′C=32
⟹C′=32C