The direction of the electric field at point P due to each plate is shown in the diagram.

Since point P lies below the plate of surface charge density the, its direction is away from the sheet which is . Hence, Electric field due to the first plate is given by . For plate second, which have a surface charge density of , Point P lies above it and direction of electric field will towards the plate which is hence, electric field due to second plate is given by

Now, for third plate Point P lies above the plate and it have a negative surface charge density of Hence, electric field will be in direction towards the plate which is , hence electric field for third plate is given by

Now, Net electric field at point P can be calculated by adding the individual electric fields, since all three electric fields are in same direction of so,

On putting the values of

So, net electric field at point P is .