It is legal in domestic ops, just so long as none of the 3 pilots exceed 8 hours. ie fly 4 hours, rotate one pilot, fly 4 more hours rotate the other pilot, fly 4 more and you're done, every pilot has 8 hours of front seat time and the plane has moved for 12 hours. I'm looking for the reg now, but feel free to contact the "East Michigan" FSDO for further.
Did it for years and it sucked, lol