Three conservative House Republicans said yesterday they have drawn up legislation to impose economic sanctions on South Africa if the government there does not take "significant steps" to abandon its official system of racial segregation, known as apartheid.

The legislation, drafted by Reps. Robert S. Walker (R-Pa.), Vin Weber (R-Minn.) and Newt Gingrich (R-Ga.), does not go as far as a bill introduced last week by a bipartisan group of about 100 mostly moderate and liberal lawmakers, including Rep. William H. Gray III (D-Pa.) and Sen. Edward M. Kennedy (D-Mass.).

But Walker said the Gray-Kennedy bill went too far by banning new bank loans, new investments, computer equipment sales and the importation of gold South African Krugerrand coins. Even if approved by Congress, he said, it was likely to be vetoed by President Reagan, who has followed a policy of "constructive engagement" with the government of South Africa.

Weber said the bill is "the first attempt by conservative members of Congress to put together a comprehensive approach to human rights . . . . Many conservatives, unfortunately, have tried to sidestep" the issue.

Weber, Walker and Gingrich and 32 other conservative GOP lawmakers sent a letter in December to South African Ambassador Bernardus G. Fourie, threatening economic sanctions unless his government took immediate steps to end apartheid.

The letter received wide notice because most previous criticism of South Africa had come from Democrats and liberals.

The bill the three GOP lawmakers drafted would deny federal contracts or economic assistance to businesses that refuse to abide by the "Sullivan principles" regarding new investments. The Sullivan principles are voluntary guidelines that promote workplace integration and other fair employment practices by U.S. companies in South Africa.

That provision would not go into effect until January 1987, and could be waived if Reagan found that the South African government had taken significant steps toward eliminating apartheid.

The legislation also would direct the U.S. representative on the International Monetary Fund to oppose IMF loans to any country that has an official segregation policy or is involved in international terrorism or illegal drug trafficking.

Unlike the Gray-Kennedy bill, which is specifically designed to force South Africa to abandon apartheid, the Walker legislation is directed at a host of countries, including the Soviet Union, that the GOP lawmakers said were human rights violators.

For instance, Walker said his bill would strengthen a ban on importing products made with slave labor and would prohibit economic and military assistance to nations that oppose the U.S. position more than 85 percent of the time at the United Nations.