The charge distribution is as shown in the figure.
Using Kirchhoff's law,
V−q1/C1+(q2−q1)/C3−q1/C1=0⇒V=2q1/C1−(q2−q1)/C3
V−q2/C2+(q1−q2)/C3−q2/C2=0⇒V=2q2/C2−(q1−q2)/C3
Therefore,
C1C3V=2C3q1−C2q2+C2q1=(2C3+C1)q1−C1q2
C2C3V=2C3q2−C2q1+C2q2=(2C3+C2)q2−C2q1
Solving these two equations we get,