Three international airlines announced Monday that they will discontinue next month their reduced "student" fares on flights from the United States to Belgium, Holland and Finland.

Pan American World Airways, Icelandic Airways and Sabena Airways all decided to abolish the fares--which allow travelers under 30 who declare that they are students to fly to Europe at $100 to $150 discounts--after a two-year price-cutting war.

Sabena, which is owned by the Belgian government, introduced student fares in May 1971 and decided to discontinue them on the government's orders. Pan Am and Icelandic officials said they decided to end the fares when they learned that Sabena would.

Pan Am drew widespread complaints from travel agencies because it set June 1 as the last day on which the fares would applicable. Many travel agents said they had already sold tickets on Pan Am flights after June 1 at the reduced student rates.

Sabena extended yesterday the date for the end of student fares until June 27 after its original June 1 date drew similar complaints. Icelandic will end the discount on June 20.

Youth fares, which apply on overseas flights to people under 24, will remain in effect.

Student fares are now applicable only on flights to Belgium, Holland and Finland.

Martin Kahn, vice-president of University Travel Company on Brattle St., said yesterday that University Travel has sold several Pan Am tickets with student discounts on flights after June 1. "We have been put in an unpleasant situation," he said.