To find the equation of a plane passing through three given points in 3D, let the three points be A(x₁, y₁, z₁), B(x₂, y₂, z₂), and C(x₃, y₃, z₃). First, calculate two vectors lying in the plane, say AB = (x₂ - x₁, y₂ - y₁, z₂ - z₁) and AC = (x₃ - x₁, y₃ - y₁, z₃ - z₁). Then, find the cross product of these vectors to get the normal vector n to the plane.
The equation of the plane is then given by:

n · (r - r_?) = 0

where r is a point on the plane, (r - r_?) is a known point (such as A ), and n is the normal vector obtained from the cross-product.