It’s been a long-held belief in the music industry that Destiny’s Child was little more than a launching pad for Beyonc Knowles’s inevitable solo career. In the proud tradition of Diana Ross, David Lee Roth, Michael Jackson, Sting and many others, she would leave her group for solo stardom. So why is Beyonc, who made a spectacular leap last year from the girl group Destiny’s Child to genuine main attraction, releasing the album “Destiny Fulfilled” on Tuesday with her old partners, Kelly Rowland and Michelle Williams? “For me it was an easy decision to record with Michelle and Kelly,” Beyonc said in an e-mail statement. “This is having fun with my best friends. There is nothing sweeter than three-part harmony.”