A consortium of three Harvard unions last night ratified a new three-year contract providing pay raises and a modified pension plan, after working under a temporary agreement since last spring.

The maintenance Trades Council, which represents Harvard engineers, plumbers and electrical workers, accepted a plan calling for wage increases of between 4 and 7 percent this year, and an additional 3 to 6 percent in each of the next two years, a union official said yesterday.

No Problems

Discussion of medical benefits, the one issue that might have provoked a major stumbling block, has been extended until December 1985.

If the two sides are unable to reach an agreement on a health plan by then, Harvard will substitute an additional wage increase in lieu of added medical benefits, Joseph W. Nigro, business agent of the International Brotherhood of Electrical Workers Local 103, said.

The contract also significantly alters the workers' pension plan, Nigro said.

Those leaving the University before the standard retirement age will not be offered cash in lieu of their pensions, he added.

Fair Deal

Nigro and Alan McWade, business manager for the Engineers Local 877, called the agreement a "fair contract."

Edward W. Powers, associate general counsel for employee relations, agreed that the contract was fair, and called the talks "very amicable."

Contract negotiations had ended in a short-term agreement last spring after seven months of dispute.

Medical Benefits

Nigro said the delaying of the health issue made the talks easier.

"It was too complicated," Powers said last night, adding, "I gave them the option of continuing for six months or putting it off."

He added that variation in pay increases is part of an attempt to make Harvard's usage levels comparable to those of other employers.