THREE REALITIES ABOUT CAMBRIDGE should be in the front of each voter's mind when the polls open tomorrow:

The city's rent-control program--which benefits the vast majority of city residents, both those who live in rent-controlled apartments and those who enjoy Cambridge's diversity--is under fire. Increasingly, big developers and landlords on the lookout for windfall profits are trying to gut the controls, though pressure on the city's housing market remains tremendous.

There is a real possibility gentrification--caused by the rapid displacement of low- and moderate-income apartment dwellers by the wealthier owners of condominiums--could irrevocably change Cambridge. The city's tight restrictions on condominium conversion are secure only by a single vote on the city council; if they are overruled as a result of the election, large parts of this city will go on, like Beacon Hill, to become home only to the fashionably rich.

Revenue losses caused by Proposition 21/2 may make life untenable for the city's poorest. Decisions will be made in the months ahead about where to cut budgets; without a progressive city council, the reductions are likely to come exclusively from the schools and human services programs, where they can be least afforded.

With those three clouds hanging over the city's future, then, tomorrow's election could be a make-or-break affair for the city. If the representatives of local business and real estate interests win, then the 1983 election may come too late to save the city. Fortunately, there are candidates running pledged to support fundamental issues of economic and social justice. They include all members of the Cambridge Civic Association (CCA) slate--David Sullivan, Francis H. Duehay '55, David Wylie, Saundra Graham, Alice Wolf, Mary Ellen Preusser, Wendy Abt and Robert White. And they also include the standard-bearers of the city's tenant convention--besides most of the CCA group, the tenant slate includes Alvin Thompson, Brian Feigenbaum, John St. George and Alfred Vellucci, the dean of the city council whose longstanding compassion has kept rent control alive in Cambridge for the past decade.

Under Cambridge's arcane but effective voting system, a "number one" vote for a candidate is especially important. Two years ago, several student groups endorsed David Sullivan number one. He has repaid their faith amply, with the most effective legislative record in the recent history of the council; among other as complishments, he has played the leading role in braking the rapid growth of condominiums. For those concerned with economic and social justice, he is the best candidate on the ticket.