You probably need to know the distance formula in 3D, or the equation of a sphere, which is .
The center on the xz plane says that the center (a, b, c)=(a, 0, c).
Plug in the three points now and solve for a, c, and r.
If you do not have the eqn of a sphere, you need to use the distance formula, setting up the distance between a point on the sphere (x, y, z), and the center (a, 0, c). Then substitution your three points for x, y, & z will give you three equations, with three unknowns, again a, c, and r.