I'm guessing them mean to put on each needle a multiple of 3. I expect this has to do with where the cables fall.
Say you've got a total of 45 stitches. If I wanted to put that on 3 needles, I'd put 15 (15 being a multiple of 3) on each needle.
Say I had a total of 48 stitches, I'd put 15 on 2 needles and 18 on the 3rd - so that each needle had a multiple of 3.
Marilyn, who is too lazy right now to pull up the pattern to see how many stitches that pattern has.