Rapper M.C. Hammer, with "Please Hammer Don't Hurt 'em," is No. 1 on the Billboard magazine pop album chart for the third straight week, but he probably won't make it four in a row. Hammer might be bumped by Madonna, whose "I'm Breathless" barrelled up to No. 2 in just three weeks. With the "Dick Tracy" movie campaign heating up, co-star Madonna's album should reap the considerable publicity benefits. Johnny Gill's album, "Johnny Gill," inched three notches closer to the Top 10--to No. 13. Hot newcomers: Steve Vai's "Passion and Warfare" (No. 19), Milli Vanilli's "The Remix Album" (No. 32), Soul II Soul's "Vol II-1990" (No. 24) and Jeff Healy's "Hell to Pay" (No. 36 ). On the pop singles scene, Roxette's "It Must Have Been Love" is the new No. 1 single.