UNITED STATES policy in Central America ought to be based on three elements. The first is support, meaning aid and the encouragement of reform, for friendly and reasonably worthy governments. The second is military nonintervention--the only position likely to bring an American president the requisite domestic leeway and international company, especially Latin company. The third is negotiation. The Reagan administration has not done all that it might on the first two points, but it has been especially deficient on the third. That is the significance of the latest meeting of the foreign secretaries of Mexico and the United States. It offered the brightest glimmer to date of American interest in the negotiating track.

To be sure, Mexico is not everyone's idea of a down-the-middle mediator. Its foreign policy often seems designed to appease its domestic left. But with its own great and growing revolutionary potential, Mexico has a plain interest in helping Central America to simmer down. This may explain why President Jose Lopez Portillo made his move last month. Perceiving that El Salvador's agony cannot be resolved within its borders, he proposed a broadscale approach. He would ease the United States' overall tensions with Nicaragua and Cuba, and add negotiations to the El Salvador mix.

For the United States, it was always a good idea to work with, not against, the Mexicans: they have a weight and an access in Central America that no administration can match. It was hard to work with the Mexicans, however, while they refused to put on their public agenda the item most troubling the United States: Nicaragua's support of Salvadoran guerrillas. Over the weekend, Mexico bit this bullet: "this supply of arms must stop," said the foreign minister. The administration responded by authorizing Mexico to convey certain proposals to Nicaragua and Cuba. The United States remains wary of Mexico's effort to open Salvadoran talks, but will reconsider the subject after the elections of March 28.

With the administration's having accepted at least two-thirds of the Mexican procedural initiative, the next moves are up to Nicaragua and Cuba. They now have a chance to start showing how well-placed is Mexico's confidence in their readiness for a live-and- let-live regional accommodation.

Is the United States ready? The administration must accept the fact that it is under a burden to demonstrate good faith too. It won't be easy. The Sandinistas at least, if not also the more practiced Cubans, are immensely suspicious folks. They may turn out to be so bent by their history and/or ideology as to be beyond accommodation.

Still, the effort is worth making. Thanks to Mexico, Cuba now stands on the verge of the grand negotiation with the United States that it has been demanding for 20-odd years. The Sandinistas have the opportunity to consolidate the humane national revolution they insist they made in 1979.

The Reagan administration seems a bit confused about its negotiating goals--whether to work closely with the Mexicans on the regional level or to draw the Soviets into a global understanding. The latter purpose is too big, too open-ended. The crisis in Central America is not without a Soviet dimension, but the administration would do better to concentrate on the Latin business at hand.