3. Real numbers $a, b, c$ are pairwise distinct, and the coordinates of the three points are respectively: $A(a+b, c), B(b+c, a), C(c+a, b)$. Then the positional relationship of these three points is ( ).
(A) form an obtuse triangle
(B) form a right triangle
(C) form an equilateral triangle
(D) are collinear
3. (D).

Let $a>b>c$, it is easy to get $AB=\sqrt{2}(a-c), AC=$ $\sqrt{2}(b-c), BC=\sqrt{2}(a-b)$. And $AB-AC=\sqrt{2}(a-$ b) $=BC, \therefore A, B, C$ are collinear.