generate 3 random variables uniformly that correspond to a hyper plane. I am doing simulation that I want a point םמ a sphere to be picked at random. I used spherical coordinates, to uniformly generate $\theta$ ,$\phi$, but I found it that it does not really uniformly generates the values along x,y,z. x and y histogram peaked at 0, while z histogram peaked at r. Is there a way to make it more random?

Why does your title say "hyper plane" but your question talks about spheres?

If you want point _on_ the sphere, read this.

If you want points _within_ the sphere, a simple solution is to generate $x$, $y$, and $z$ uniformly in $[-r,r]$ and simply reject all points that are outside $x^2+y^2+z^2>r^2$. Also see this related (identical?) question.