Help please -- Problem of hydrostatics force in flat surfaces

[Queren Suriano]

Homework Statement

Homework Equations

The Attempt at a Solution

I could know the pressure in point B

If I calculate the heigh of pressure I got:

But I don't know where is my free surface in the container 3, is it down??

I don't know how to keep doing it...

 

[haruspex]

On the face of it, you need to integrate over the sloping surfaces to get the vertical force (because the pressure varies with depth).
Let me suggest a simpler approach.
Consider an object of known volume sitting at the bottom of a tank of fluid of known density and depth. The shape is arbitrary, except that the base is flat and sits squarely on the flat bottom of the tank. The contact surfaces are not perfectly smooth, though, so fluid can readily seep under the object.
What is the net upward force on the object from the fluid?
What is the upward force on the object arising only from the fluid that seeps under the object?
What can you conclude about the downward force on the object only arising from the fluid elsewhere?

Can you see how this would help you figure out the force contribution from each compartment in your problem?