Logical thinking problem, trouble solving

The four rooms are set to 1, 2, 3 and 4. Three rooms can be opened at the same time. There are four possibilities, namely 1, 2, 3; 1、2、4, 1、3、4; 2,3,4, so the first one can be set as 0 -6, the second one as 6- 12, the third one as 12- 18 and the fourth one as 18-24. Each room lasts for 6 hours, so the cumulative time is 18 hours.