Britney Spears’ new album is still set to debut at No. 1 on the U.S. pop chart when sales data are released next Wednesday, according to latest estimates. “Circus,” the pop singer’s latest comeback attempt, sold about 330,000 copies in the first three days since it went on sale on Tuesday. The tally is derived from sales at Trans World Entertainment, Best Buy, Circuit City, iTunes, Starbucks, Borders, Target and Anderson Merchandisers. Billboard estimates the eight merchants account for about 80% of all U.S. album sales. “Circus” would be Spears’ fifth No. 1 album. Among female artists, only four have racked up more chart-toppers: Barbra Streisand (eight), Madonna (seven) and Mariah Carey and Janet Jackson (both with six).Spears’ last release, “Blackout,” bowed at No. 2 with 290,000 copies in November 2007.