3. Consider the three diagonals on the three pairwise adjacent faces and the space diagonal of a rectangular cuboid (a total of four line segments). Then the correct proposition is ( ).
(A) There must be some three segments that cannot form the three sides of a triangle
(E) Any three segments can form a triangle, each of whose interior angles is acute
(C) Any three segments can form a triangle, one of which must be an obtuse triangle
(D) Any three segments can form a triangle, which is either "acute" or "non-acute," depending on the length, width, and height of the cuboid, and cannot be determined
3. (B).

Let the three dimensions (i.e., length, width, height) of the rectangular prism be $a, b, c$, then the four line segments are $\sqrt{a^{2}+b^{2}}, \sqrt{b^{2}+c^{2}}, \sqrt{c^{2}+a^{2}}, \sqrt{a^{2}+b^{2}+c^{2}}$. By the triangle existence theorem (the sum of any two sides is greater than the third side) and reverse reasoning (inequality of sides squared), it can be known that: any three line segments can form a triangle. Then, by the corollary of the cosine theorem (determining the relationship between the internal angles of a triangle and $90^{\circ}$) and reverse reasoning, it can be known: each triangle is an acute triangle. (If the geometric meaning of the four line segments is used, this problem cannot be solved, it should be transformed into an algebraic problem)