In the third test case:

<1, 2, 3> , <1, 3, 2> ⇒ One pair of students go to construction.

<1, 2, 3> , <2, 1, 3> ⇒ One pair of students go to construction.

<1, 2, 3> , <2, 3, 1> ⇒ No pair of students go to construction.

<1, 2, 3> , <3, 1, 2> ⇒ No pair of students go to construction.

<1, 2, 3> , <3, 2, 1> ⇒ One pair of students go to construction.

<1, 3, 2> , <2, 1, 3> ⇒ No pair of students go to construction.

<1, 3, 2> , <2, 3, 1> ⇒ One pair of students go to construction.

<1, 3, 2> , <3, 1, 2> ⇒ One pair of students go to construction.

<1, 3, 2> , <3, 2, 1> ⇒ No pair of students go to construction.

<2, 1, 3> , <2, 3, 1> ⇒ One pair of students go to construction.

<2, 1, 3> , <3, 1, 2> ⇒ One pair of students go to construction.

<2, 1, 3> , <3, 2, 1> ⇒ No pair of students go to construction.

<2, 3, 1> , <3, 1, 2> ⇒ No pair of students go to construction.

<2, 3, 1> , <3, 2, 1> ⇒ One pair of students go to construction.

<3, 1, 2> , <3, 2, 1> ⇒ One pair of students go to construction.