Knowing the trailers OEM tire size and load range would help as were guessing here on what size and load range would work the best for your particular trailer.
Some tri axle trailers can have 5.2k or 6k or 7k axles. And some tri axles have came with 15" wheels.
This will help in determining what size and load range tire would be better.
Example....you sure don't need a 3750 lb rated load G tire on 5.2k axles nor would a load E LT tire at 3042 lb work best on a 7k axle.
And I sure wouldn't put any brand ST tire on a tri axle trailer.