Question: How can get a point M(x,y,z) in side the triangle TAD?

I have triangle TAD. Now I want to take any point inside the triagle TAD. How can I take it? I tried

A := [1, 2, 3]; 
DD := [-2, 1, 0]; 
T := [1/2, 1/2, 3/2]; 
P := Plane(T, A, DD); 

I got the quation of the plane TAD is

-3*x-3*y+4*z = 3

With the point P(x,y,z). What is the condition of x, y, z?

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