What is called a number that can add 2 or 3 numbers and have the same sum?

A number that can add two or three numbers and yield the same sum is often referred to as a "constant" in the context of mathematical relationships. In specific cases, it may also be a "balanced" number, especially in puzzles or problems where the sums remain unchanged regardless of the quantity of addends. However, there isn't a widely recognized specific term for such a number outside of these contexts.