Unfortunately there's conflicting information. I'm posting the link to my claims source of information below.
Different sources have different information though and some sources support your claim... Including Wikipedia. I'm going to give on this one as it seems most likely that you are correct.
I don't have time to check and update my comment now. I'll return in the near future with citations to either confirm what you are saying or continue the discussion. 🙂
Multi-use allows unlimited entry for 6 months with each entry being a maximum of 60 days by default.
Considering you can file a 30-day extension to any non-expired entry stamp at immigration, the actual correct answer is if optimally used they can be in Thailand for up to 9 months on this visa.
respect about bringing citations. I have had it clearly explained to me a couple times that The number is 3, however, once you gave citations I double checked and your info is correct. And about to go and edit any comments I made with incorrect information.
Considering that he was smart enough to ask a good question and learn that the correct answer is actually 9 months if optimal utilized... He very much should be allowed to be out without his parents.
Unfortunately, that answer is incorrect. This is a multi-entry visa which allows three utilizations of a base of 60 days. All entries (as long as they have not yet expired) are capable of being extended an additional 30 days.
(I am leaving incorrect information here so that readers can see how I was corrected below)
So far I have not seen anybody else post the correct answer with explanation here.
Your multi-entry Visa allows you unlimited entries with a max duration of 60 days each for a period of 6 months from your first use of it. As long as you go into the immigration office before that 60 days ends you are capable of extending each stay by another 30 days.
Additionally, and each calendar year, most western countries are allowed three fly-in 30 day automatic entries, and two walk in 30 day automatic entries. Each of those can also be extended by 30 days if you go into the immigration office before they end.
Your multi-entry pass will allow you (with optimum utilization) 9 months, and if you choose to use your automatic entries, you might be able to wiggle another 14 months if you time it right in the calendar. (The auto entries apply in any calendar year and renew each year)