“Three times a bridesmaid, never a bride” dates to about the sixteenth century. It was believed that if young maiden who had been a bridesmaid three times was unable to catch the eye of unmarried males, then she never would. But, if she served seven times as a bridesmaid, the spell was broken and the woman was thought to be a sure bet for marriage.
From: Lee, Vera. 1994. Something Old, Something New: What You Didn’t Know about Wedding Ceremonies, Celebrations, and Customs. Naperville, IL: Sourcebooks, Inc.
She'd better gather up a big group of friends indeed!