Each band watches a different octave of the note: the top band follows the fundamental and the lower bands follow the 2nd and 4th harmonics. Higher bands drift faster for the same error, so use them for the last fraction of a cent.
Just intonation tunes each note to a pure ratio from the key you pick. Major thirds come out about 14 cents flat of equal temperament and fifths about 2 cents sharp, which is how many steel players set their thirds.