How are you supposed to do it? Analog parts? Using a micro? a PC? You could build a set of filters, 2 bandpass, a low pass, and a high pass? You could do a _very_ easy modified FFT in software (since you know the frequencies, you dont need to compute the whole thing). You could use three high pass filters, then use logic to deduce which signal showed up ...
there are lots of solutions to this problem ... please post some more information about how you plan to do it, what you are supposed to use, etc.