DOWNLOAD THIS: The first thing we note, on this turnover Tuesday, is the appropriately explosive #1 bow of "Bang Bang" (Lava/Republic), the Jessie+Ariana+Nicki bombshell that's invited multiple comparisons to "Lady Marmalade" and is slated to light up the VMAs in less than a month. Epic's Meghan Trainor continues to gather momentum with the sassy "All About That Bass." On the album side, Island's Shawn Mendes holds off two classic-rock icons, EPC's Eric Clapton (whose J.J. Cale tribute boasts guests like Tom Petty and Willie Nelson), and the latest from Warner Bros.' Petty himself, which is set for a big chart debut next week. Rock is also repped by metal mongers Theory of a Deadman (who snag another classic-rock luminary, Alice Cooper, to guest on one track). Capitol's 5 Seconds of Summer remains on the leaderboard mere hours from their huge #1 album chart bow. OVO/WB's EDM/hip-hop project PARTYNEXTDOOR, meanwhile. also makes a strong entrance on the album side, powered by a single featuring Drake. (7/29a)