- Read your file using wavread
- Specify the filter using fdesign (FDesign Help) where you need 3 filters lowpass, bandpass and High pass. (You may need to decimate)
- Use design to actually design the filter from above specification. You need to specify & design thrice as you have 3 channels. This can be made into a function and wavread output can be passed to this.
- Finally use filter command to actually filter the data. (Again 3 filters)
- Now you have 3 outputs which belong to 3 frequencies. (Use Matlab help its really good)
PS: I assume you have Signal Processing Toolbox.