oh wait, i think i understand now. yes, that sounds pretty weird. how about just getting another matching 3-way tower and having a nice stereo system? you don't really need a sub if your 3 way has decent bass.
wait- let me make sure i've got your plan right - you want to use the 3 way as a centre channel and sub, and use the bookshelves for left and right channels, right?
cos having four channels (centre, sub, left, right) wouldn't do you much good, since you couldn't play anything in surround sound, you'd be missing the back two channels. besides, i don't think you can channel the subwoofer channel and the centre channel into one speaker. correct me if i'm wrong.