3. Given that $a$, $b$, and $c$ are any real numbers. Then $(a-b)^{2}-4(a-c)(c-b)$ is always ( ).
(A) greater than 0
(B) equal to 0
(C) less than 0
(D) greater than or equal to 0
3. D.
$$
(a-b)^{2}-4(a-c)(c-b)=(a+b-2 c)^{2} \text{. }
$$