its a normal series combination of capacitors,only the inner plates are placed differently,one in the beginning and one in the middle,will it not alter the charge distribution ? that is if we will start the charge distribution on the plates from left to right,thanks.
I think that you should forget about categorizing this arrangements of plates as being in parallel or in series and look at the problem in a different way.
Calling the plate on the left $1$, the potentials of plates $1$ and $3$ are the same and they are electrically isolated.
This tells you something important about the magnitude of the electric field between the plates $1$ and $2$, and between $2$ and $3$.
From this you can get the relative distribution of charges on the plates and hence solve the problem.
Another way is to find the distribution of charges which minimises the electric potential energy of the system of charges on the plates.