I'd second what Shadorne said. If it's not implemented properly, then the more variables you introduce, the greater that something will mess things up. As a matter of practicality though, more speakers may be needed in a room with a lot of people.
If your L and R speakers are set up properly, and you're sitting in the right spot, you don't need a centre speaker. There should be a stereo image of somebody speaking in between the L and R speakers. That's what a stereo does in the first place and why two speakers are used, i.e. to produce a "stereo" image and a soundstage. Otherwise, just use a single speaker in mono.
Also, if you have large full range speakers with deep bass response, you don't need a sub.
By the way, two-way and three-way speakers, as in your thread title, is not the same thing as using two or three speakers.
If your L and R speakers are set up properly, and you're sitting in the right spot, you don't need a centre speaker. There should be a stereo image of somebody speaking in between the L and R speakers. That's what a stereo does in the first place and why two speakers are used, i.e. to produce a "stereo" image and a soundstage. Otherwise, just use a single speaker in mono.
Also, if you have large full range speakers with deep bass response, you don't need a sub.
By the way, two-way and three-way speakers, as in your thread title, is not the same thing as using two or three speakers.