Find the rule: 1, 2, 3, 2, 3, 4, 5, (), (), (), what numbers should be filled in the brackets? Why?

Find the rule: 1, 2, 3, 2, 3, 4, 5, (), (), (), what numbers should be filled in the brackets? Why?

1、2、3、2、3、4、5、( 3)、( 4)、( 5)
The numbers are arranged as follows:
1 2 3
2 3 4 5
3 4 5 6 7
4 5 6 7 8 9