For Paul McCartney's sake, the third time better be a charm. The Beatles singer is set to walk down the aisle and make longtime girlfriend Nancy Shevell Mrs. McCartney #3. The 51-year-old brunette beauty began dating the British legend four years ago. Now, they plan on taking their relationship to the next level, according to sources. An insider tells People, "Nancy and Paul are getting married. Ring and all -- very exciting." Sure enough, McCartney's rep confirmed the pending nuptials. Although 17 years separate the two, sources close to the couple say that they're a match made in heaven, "They have the right chemistry. They're both cool, chilled out and optimistic." Hopefully, McCartney's union to Shevell will be more like his first marriage to Linda and less like his relationship with Heather Mills. Unlike Mills who was constantly at odds with McCartney's grown children, Shevell has apparently won over the lot. His five children include: 48-year-old Heather, 41-year-old Mary, 39-year-old Stella, 33-year-old James and 7-year-old Beatrice. Daughter Stella, one of Mills fiercest opponents, is said to be "thrilled for her father."