Yup, that's my understanding as well - where possible, leave an open string to ring whilst playing the other two notes.  Where it's not possible and all 3 notes require to be stopped, I try to get the lowest two notes double-stopped, sounding first, then roll over onto the middle and top note also as a DS, keeping the now sounding lowest note stopped, but un-bowed.  I don't know if that's the right approach or not, obviously it keeps the middle note being played all the time - but it sort of works for me (on the odd occasion when I actually succeed in doing it! )