WITH ONLY A LITTLE over three weeks remaining in what, only light years ago, seemed an endless primary season, suspense is fast becoming an endangered species. Tuesday's primary results from Indiana, North Carolina and Tennessee significantly strengthened the positions of the two front-runners, Jimmy Carter and Ronald Reagan.

With time no longer neutral, let alone an ally, Sen. Kennedy and former ambassador Bush, the only and barely surviving challengers in their parties, must base their strategies on their opponents' quickly making large and serious mistakes. For Sen. Kennedy, who entered the race last November as the overwhelming favorite for the nomination, the frustration must be considerable. Survey after survey of voters in primary states reveal that the senator's message is popular. Voters in Democratic primaries from New Hampshire to Pennsylvania seem to support wage and price controls and almost uniformly give negative ratings to Mr. Carter for his handling of the economy and inflation. But while these Democratic voters apparently agree with the Kennedy message, they have been rejecting Mr. Kennedy as the messenger. The Massachusetts Democrat has been consistently rated unacceptable as a president, according to these same surveys, by nearly one out of three Democratic voters.

Indiana, on Tuesday, gave no comfort to the authors of the Industrial State strategy for Sen. Kennedy. The president received 68 percent of the vote there to go with his 75 percent in Tennessee and his even 70 percent in North Carolina.

Former governor Reagan marched triumphantly through all three states as well. Mr. Reagan's long lead in committed convention delegates is even more impressive than the president's. And the Democratic race is not exactly nip and tuck.

While suspense may be scarce, drama will abound. Both parties still must go about the sensitive and difficult task of trying to bring autumn unity out of spring disorder. The Republicans would appear to have the easier job. The Reagan-Bush contest has been remarkably free of bitterness, although any more calls for Mr. Bush's withdrawal from his former (and vanquished) opponents could jeopardize that record of harmony. Mr. Reagan, himself no stranger to challenging front-runnters, has been quite circumspect in his own statements about the Bush candidacy, which demonstrates both sense and sensitivity.

Democrats, seemingly as always, have both a greater propensity and the experienced personnel needed for a real civil war. The very fact that there has not been a single divisive issue such as civil rights or Vietnam does not seem to have diminished the capacity for bitterness between the competing camps. One valuable quality of political leadership is the ability of the victor both to conquer and to enlist the vanquished. Mr. Carter and Mr. Reagan will probably have ample opportunity to demonstrate their respective political skills in the weeks and months ahead. And how diplomatically and effectively they perform will provide some real indication as to what kind of campaign each will run and what kind of president each might be beginning next January.