I read that the immigration office is at Bumrungrad the third Tuesday of every month, where in the Hospital is it located?

Here's the link to the hospital's website map page. Bumrungrad Hospital Map

They're on Soi 3 which you can get to from either Sukhumvit Road or Phetchaburi Road; you can't miss it! Know that Soi 3 is a one-way road running south from Phetchaburi Road.

Tomorrow, Tuesday the 19th; Mobile Immigrations will be there (I think it's on the 10th floor). They’re open from 8:30AM to 4:30PM (they do have an hour lunch break though when they’re closed).

I'm going there to turn in some 90 day reports for acquaintances and to scope out how things appear to be working at these mobile locations.

Hope it helps!