i got

1
1
2
2
3
3
4
4

and i want them to become

1
2
3
1
2
3
4
5
6
4
5
6
7
8
9
7
8
9
10
11
12
10
11
12

ie
1 expands to 1 2 3
2 expands to 4 5 6
3 expands to 7 8 9
4 expands to 10 11 12

pls help