Welcome to the community, @Kristina_Lucarelli! :grinning_face_with_big_eyes: And welcome to the wonderful world of lookup fields.
Lookup fields create (and return to formula fields) arrays of values, not individual values. Because of that, they’re the source of much confusion in terms of how to use them. Long story short, you can’t add a collection of arrays in the same way that you would add a group of individual values.
On a side note, the correct way to pass things to the SUM() function is by separating the items with commas:
SUM({Field 1}, {Field 2}, {Field 3})
That’s the same as writing this:
{Field 1} + {Field 2} + {Field 3}
While that doesn’t work around the lookup field issue, we can use the SUM() function in a different way to solve this problem.
In addition to adding comma-separated items, SUM() can also add the contents an array. Knowing that, you could SUM() the array from each lookup field, and then add those sums together for the final result, like this:
SUM({B-Combined TRT}) + SUM({A-Unique TRT}) + SUM({B-Unique TRT})
That should produce the output you’re seeking.
