This is the rarest of Oscar oddities: a bona fide three-way race. Julianne Moore's bushel of critics awards still give her an outside shot, even though Nicole Kidman and Renée Zellweger have won the biggest lead-up awards. First, Kidman picked up the Golden Globe and the BAFTA prize, but then Zellweger countered with a victory at SAG. But expect Kidman to come back strong on Sunday. First of all, the SAG voters usually award bigger commercial successes (''Chicago'' has earned about four times as much as ''The Hours'' at the box office). And as Nicole knows so well from last year, when her performance in ''Moulin Rouge'' lost to Halle Berry's turn in ''Monster's Ball,'' it's tough to win an Oscar for a musical.