In set theory, the union of three sets that each contain only one would result in a single set that itself only contains one. In other words:
{1} ∪ {1} ∪ {1} = {1}
Alternatively, you can replace the numbers with “Father”, “Son”, and “Holy Spirit” and get:
{Father} ∪ {Son} ∪ {Holy Spirit} = {Father, Son, Holy Spirit}
That’s probably the most relevant mathematical representation of the Trinity, since the Trinity is, in fact, a perfect union. This, of course, has one set containing three distinct values. That’s similar to how the Trinity is one being containing three distinct persons.
Basically, by selecting standard arithmetic addition, you assume no union, which is a faulty representation of the Trinity. As a result, set theory, which does have a union, is perhaps more apt for this kind of analysis than the arithmetic we learn as three years olds.