Gravity

[_Aka_]

A set of sphere released at the same time should collide at point C

Just in case it is your first project with such physics, keep in mind that the reality will be different from theory, because of floating point imprecision, error accumulation, and 3d meshs being different from the mathematical surface they are meant to represent (because of tesselation). So having precise physics might be difficult, especially if you are networking it, but it is worth giving a try.

Here is a clip of a test scene I made, that does what I think you want: an U-shaped tube made with Curvy, and two balls dropped from opposite sides.