Bing seams to be a little difficult for me..One of my older sites does not get much traffic from google or yahoo, but seams to get allot from bing..I dont know how or why that is..lol. As for Yahoo, from my experience you need allot of backlinks. Some decently optimized pages and a high number of relevant backlinks will have you ranking for similar terms you never even thought of..lol
I don`t know bing, to complicated, slow, and bad rank algorithm. For Yahoo! is important to have exact match of keyword and lot of backlinks. Regards, Fluor