There are a couple of online calculators you might be able to use to justify how much water they are using. You could use these ammo to justify their consumption vs yours. Here's one I found online: https://www.home-water-works.org/calculator
Another option might be to divide the total bill by the total people in the triplex. If there are 6 in that unit, 1 in yours, 3 in the other, that's a total of 10 people. At minimum they should be responsible for 6/10ths of the bill, right? Without calculating the pool. Just a thought.
Might be worth the upfront expense of separate metering. Know a good contractor who can give you an estimate of cost?