Never heard this, I have heard the three sides not divisable between them or by a number. something like 15 x 21 x 7, where 3 or 7 would be a common or 12 x 16 x 7 where 4 would be the common
axiom_man,
You are supposed to avoid having any of the dimensions divisible by the same number if at all possible. Since it sounds like you will be picking the dimensions then you can avoid having them all divisible by 3 (15.5 is nearly divisible by 3). I would just change them all slightly to avoid this issue, which can cause acoustic problems.
