“American Idol” is sprinting to the finish line, racking up hefty year-to-year gains Wednesday night and lifting Fox into a near tie for the May sweep demo lead. As the third week of the ratings period came to a close, “Idol” and the season finales of “That ’70s Show” and “Stacked” put Fox in the winner’s circle once again. Night also saw ABC’s “Lost” picking up more steam as well as strong season-enders for UPN’s “America’s Next Top Model” and the WB’s “Smallville.” From 9 to 9:30, the six broadcast nets combined for a 63 share in adults 18-49 – their highest for any half-hour so far this sweep. They figure to top that next Wednesday, though, when the finales of “American Idol” and “Lost” go head-to-head from 8 to 10. Meanwhile, with the “Idol,” “House” and “24” finales still ahead for Fox and many of the big CBS guns already fired off, Fox is expected to win the month in 18-49. Through three weeks, though, the Eye still edges Fox (4.3 rating/12 share to 4.2/12), and CBS is projected to win comfortably for the month in adults 25-54 and total viewers.