I noticed there is not much discussion on Thammasat University here, which is interesting because it's now triple accredited.

I'm Thai but I have been living in the UK for several years now. I plan on returning to my home country to do an MBA. Can anybody please advise about the school quality compared to other schools like Sasin and Chula. Thank you for your time.

It's a good school, 'quality' wise on par with the other strong schools in the country. I think Sasin generally has a more international profile, while the other two are definitely focused domestically, but this might be changing. For students, I don't think triple accreditation necessarily adds much value over having just one or two, but it does add value in terms of international perspective.

If you have a sense of your goals, you can look at LinkedIn. Look at people who are working in the companies / positions you'd be aiming for and see where they did their MBAs. There might be differences across sectors.

It also looks like Chulalongkorn's b-school also just got EQUIS accreditation, which is pretty cool:

https://find-mba.com/news/2019/04/mba-accreditation-awarded-to-chulalongkorn-business-school-indian-institute-of-management-indore-and-others

Forgive me for asking but does this mean that Chulalongkorn has two business schools (the 'business school' and then Sasin as well?)

Two separate business schools, part of the same university.

Sasin has typically been more outward / international facing - it was founded in partnership with the Wharton School.

I think they had the same quality, but Sasin before you will graduate, you need to complete 

1 project and 1 IS.  

Interesting info here, as I'm also looking into MBA options in Thailand. Currently looking into Sasin and AIT, but Thammasat Business School, looks like a solid option.