Which set of numbers does -3 go into?

Answer 1

To any set that contains it!

It belongs to {-3},

or {45, sqrt(2), -3, pi, -3/7},

or all whole numbers between -43 and 53,

or multiples of 3,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

or square roots of 9,

of fourth roots of 81,

etc.