3 + 2 is just 5. But people will sometimes write 3i + 2j for the point or vector (3, 2), where i is the vector (1, 0) and j is the vector (0, 1).

In your example 3 and 2 belong to different dimensions. you could write "3x + 2y" to express 3 steps along the x axis and 2 along the y axis. Just writing "3+2" would lead to the expectation that 5 could be calculated as reasonable result, which would not be the case.

You can absolutely deal with complex numbers with the (x,y) notation