平面一般方程为ax+by+cz+d=0,求出a,b,c,d即可

别人博客的文章，是用C++求的
http://blog.csdn.net/hoya5121/article/details/1484365
//已知3点，求平面方程
BOOL CGe::PanelEquationFromThreePt(CPoint3dArray ptArr,
double &a,double &b,double &c,double &d)
{//from http://blog.csdn.net/hoya5121
CPoint3d p1,p2,p3;

if(ptArr.GetSize() < 3)
{
return FALSE;
}
p1 = ptArr[0];
p2 = ptArr[1];
p3 = ptArr[2];

a = ( (p2.y-p1.y)*(p3.z-p1.z)-(p2.z-p1.z)*(p3.y-p1.y) );
b = ( (p2.z-p1.z)*(p3.x-p1.x)-(p2.x-p1.x)*(p3.z-p1.z) );
c = ( (p2.x-p1.x)*(p3.y-p1.y)-(p2.y-p1.y)*(p3.x-p1.x) );
d = ( 0-(a*p1.x+b*p1.y+c*p1.z) );

return TRUE;
}