Xiaoming will meet Xiaogang for the first time in the library on July 6th.

The least common multiple of 3 and 4 is 12.

Xiaoming will meet Xiaohua for the first time in the library on July 12;

The least common multiple of ∵3 and 2 and 4 is12;

On July 12, all three of them went to the library to read again.