The operation of addition is both commutative and associative. Therefore, any of the three addends can come first, and then either of the remaining two can come second.
There are three ways to select the first addend. For each of those ways there are two ways to select the second addend. Three times two is six, and for each of those six ways, there is one way to choose the last addend. Therefore there are 6 ways to solve the problem.
However, there is only one correct answer. In that sense, there are only two ways to solve the problem, correctly or incorrectly. On the other hand, since there is only one correct answer but an infinity of incorrect answers, there are an infinity of ways to solve the problem.
