Well, 5 toons can EASILY 1-shot other people at will. 3 toons may not be able to do it as readily.

If you're planning on doing instances as well, I'd go with 5 boxing. I have a pretty good system running 2 wow's, and I bought some cheapo used 1.7ghz athlon PC's, and tossed in the lowest video card that would run WoW (like $35 each). It was fairly cheap, and works for me.