Splitting channels is something you do after it has been separated. "Splitting"is to break up the separated file) into individual files. What you want, is the separation process.
Select all, Copy, Past into an new channel, make 3 more copies or duplications of that. Then adjust each one so that you have more of a tone like (mostly what would print white). Then another that would be for what would mostly print Medium tone. Then another for the darkest tone. Also knock each of those out of the others.
Then, once completed, add extra space at the top and bottom, and add in a center line...and registration marks. Be sure those are copied and pasted into each channel in the exact same location on each.
Then, see the attachment.