The extra time is exactly 1 second for the 1st lighthouse, exactly 2 seconds for the 2nd, and exactly 3 seconds for the 3rd.
In summary:
- The first light shines for 3 or 4 seconds, then is off for 3 seconds.
- The second light shines for 4 or 6 seconds, and then is off for 4 seconds.
- The third light shines for 5 or 8 seconds, then is off for 5 seconds.
What is the expected value of the first time all the lights will be off at the same time?