So let's call it 33 units on each side, since that's a multiple of 3. I assume "stack" means a vertical section of meters, if you can't use 6 position stacks due to space constraints, do you want to use 3, 4, or 5? And does each stack get fed with 3 phases, or with just 1 phase (which alternates among stacks)?

3 position stack, 3 phases per stack: no problem, use 11 stacks per side.

3 position stack, 1 phase per stack: simplest would be to use 12 stacks per side, leave one blank space in each of the last 3 stacks.

4 position stack, 3 phases per stack: 9 stacks gives you enough spaces, but each stack will have one phase repeated. If the repeated phase is the same in each stack, then you'll be unbalanced. So you'll want adjustable stacks that let you change with phase gets repeated. You change 6 of the stacks, so that you end up with 3 stacks have two AB positions, 3 stacks having two BC positions, and 3 stacks having two AC positions.

4 position stack, single phase per stack: use 9 stacks, no problem, again leave one blank space in each of the last 3 stacks.

5 position stack, 3 phases per stack: 7 stacks give you enough spaces, but each stack will have one phase non-repeated. If the non-repeated phase is the same in each stack, then you'll be unbalanced. So again you'll want adjustable stacks, and you'll adjust 4 stacks so that 2 stacks have a single AB position, 2 stacks have a single BC position, and 2 stacks have a single AC position. The last stack will only have 3 meters, so just be sure those 3 are balanced.

5 position stack, 1 phase per stack: 7 stacks give you enough spaces, and the first 6 of those will result in a balanced system. The last stack with just 3 meters in it will be an issue, but that's the best you can do without buying 2 more stacks. Just be sure that this seventh stack is on different phases on each side. The system will be slightly unbalanced.

Cheers, Wayne