The funny thing about Google/Yahoo/MSN is the SEO itself. If you optimize your site for one of them, the other two may not work so well. That's why it's the best to concentrate on Google (because of the traffic).
I have same rank for a keyword on google, yahoo and guruji. Google sends 80-90% traffic while guruji contributes more than yahoo. The site is a region specific site hence might be getting more hits from a regional SE.