Victor Korchnoi increased his margin over Boris Spassky to three points in the seventh game of their chess match in Belgrade, Yugoslavia, making it almost certain that he will be the challenger in next year's world chess championship match.

Spassky, a notoriously slow starter in match play who has rescued himself brilliantly in the late stage of two matches earlier this year, now faces an almost impossible task.The first player to score 10 1/2 points will win.

With the score at 5 to 2 in Korchnoi's favor, all he has to do is draw 11 more games in the 20-game match. Spassky must prevent this, while winning three to equalize or four to pull ahead. It is theoretically possible but not likely to happen.

Korchnoi, who defected from the Soviet Union in July 1976, in now within sight of the goal he had been pursuing single-mindedly since then: to challenge world champion Anatoly Karpov for his title. Korchnoi lost a 24-game match with Karpov by a single point three years ago, and since then he has claimed that the Russian authorities put him under heavy psychological pressure to lose.

To get another crack at Karpov, he has played a series of matches this year which amounted practically to a single-handed grudge fight against the Soviet chess establishment. He already has demolished Soviet grandmasters Lev Polugaevsky and Tigran Petrosian and now seems well on the way to doing the same to Spassky.

Spassky, a former world champion who lost his little to Bobby Fisher in 1972, adjourned the seventh game Friday after 42 moves, but played only six more moves, after the game was resumed, before resigning.

Game 8 will begin today with Spassky playing white.