As they await the birth of their third child, Mark Wahlberg and his longtime girlfriend, Rhea Durham, are finally getting married. “They are planning to spring the wedding on most of their pals,” says an insider. “They’re going to invite people over for a pool party and surprise them with the wedding.” The friend adds that the nuptials could happen at any time. “No one is going to know until the last possible moment. It could happen in a few days or months from now.” Although Mark gave Rhea a massive diamond ring when their daughter, Ella, was born back in September 2003, the couple never officially announced their engagement — and they still haven’t. Mark, 37, and model Rhea, 30, first started dating in 2001. They have two kids — Ella, 4, and Michael, 2 — and are expecting a boy in September.