Controlling audio high/mid/low in frequency domain Is it a realistic mapping to divide the frequency bins in 3 parts, high/mid/low frequencies?

How can these bins then be manipulated to control the level of these bands in the time domain?

Is it sufficient to adjust these bin segments and do an iFFT to time domain?
while you could do this in the frequency domain, there are many problems with this approach, some of which are outlined here.

A better approach is to use a time domain filter. There is a tutorial here.