The Reagan administration says last summer's drought took such a large bite out of the economy that it will add about $3 billion to the reductions needed in next year's federal budget deficit.

Joseph R. Wright, director of the White House's Office of Management and Budget, said Tuesday that the administration now believes the fiscal 1990 deficit will have to be slashed by about $35 billion to comply with federal law.He said that in the 1990 budget the administration currently is writing, about $15 billion in reductions probably would come from various benefit programs for the sick, farmers, federal retirees and others who automatically qualify for government help.

He said the rest would come from a combination of reductions in other federal spending, sales of federal assets and increases in user fees.