Vector3d v1 = new Vector3d();
Vector3d v2 = new Vector3d();

v1.sub(p1, p2);
v2.sub(p3, p2);

Vector3d n = new Vector3d();
n.cross(v2, v1);
n.normalize();

Vector3d p = new Vector3d(p1.x, p1.y, p1.z);      

double d = -p.dot(n);