As Sheryl Crow might sing to the Big Brother housemates, change will do them good. In fact, Crow can tell them in person when she appears for a surprise concert Aug. 21 in the BB3 backyard. The eight remaining contestants — confined since July 6 in a house with no music or TV — will be treated to two songs from Crow’s current C’mon, C’mon as well as ”All I Wanna Do.” ”Last year, we had the Survivors come in [to BB2] so we certainly wanted to have another special event,” says exec producer Allison Grodner. ”It’s going to be a complete surprise for them.” Not nearly as surprising as CBS’ coup in persuading Crow to soak up the BB3 sun. ”She’s a fan of the show,” says Interscope spokesman Dave Tomberlin of the unusual gig. ”And when was the last time she got to play for eight people in a backyard setting?” Next stop: Home Shopping Network?