is 3i a whole number, a natural number, an integer, or something else?

 

$$The imaginary number system is based on $i=\sqrt{-1}$\\
So \\
3i = \sqrt9\times \sqrt{-1} = \sqrt{9*-1} = \sqrt{-9}$$

.

3i is an imaginary number.

It is the square root of -9.