Yahoo and Bing are almost insignificant with respect to how much traffic they can drive versus how much Google can drive. Google is the 800-pound gorilla.
If your site ranks well in Google, it will likely rank well in Yahoo and Bing. There really aren't any other search engines in the global market that are worth focusing on. Some countries have their own search engines that do a good job competing with Google in the local market, such as Yandex in Russia and Baidu in China, but none of the local search engines compete well on a worldwide basis.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin