Victor Korchnoi, who wants to be the next world chess champion, took another step toward his goal yesterday in Belgrade, Yugoslavia, winning the third game of his match with Boris Spassky. In the first three games, Korchnoi has rolled up a score of 2 1/2 points to 1/2 for his opponent. The first player to score 10 1/2 points will have the right to challenge world champion Anatoly Karpov next year.

Expert observers at the match said that Spassky, a former world champion who lost his title to Bobby Fisher in 1972, seemed an equal opponent for Korchnoi only in the opening phase of the game, where the possibilities have been thoroughly analyzed by generations of grandmasters. In the middle game, where much more depends on the player's own initiative, Korchnoi took control and dominated play until the end.

A two-point lead at this phase of the match is overwhelming though perhaps not decisive. Spassky had a similar edge over Fischer going into the third game of the 1972 match which he ultimately lost.

In two earlier matches this year, Spassky got into trouble in the first half, then dramatically came from behind in the late stages, winning the right to play this final challengers' match with Korchnoi.

Korchnoi's two earlier matches, against Russian grandmasters Lev Polugaevsky and Tigran Petrosian, were both runaway victories. At this stage, his performance in the elemination matches is beginning to look nearly as spectacular as the brilliant series of victories that preceded Fischer's 1972 championships match.