In the wake of the expulsion of three American diplomats from Nicaragua, the United States would be wise to keep open its lines to the Nicaraguan regime. Any precipitate action - and there is talk in Washington of severing diplomatic ties with Managua - could prove counterproductive. However difficult relations between two countries become, the better part of diplomatic wisdom is always to have someone on the spot able to continue reporting on what is going on and to serve as a channel of communication for a possible improvement of ties.
The American public may never know the facts surrounding the case. The Sandinista government charges one of the three with a plot to poison the Nicaraguan foreign minister. On the face of it the charge does seem ''absurd,'' as the US Embassy put it. The United States does not conduct foreign policy in this manner. Certainly radical leftists (and rightists for that matter) are capable of fabricating conspiracies, especially one so crude. Given the regime's inordinate suspicion of the US, Nicaraguan officials could have simply blown up routine US information-gathering into something politically exploitable. Or perhaps they genuinely mistook what is the legitimate political reporting of any embassy, including contacts with various individuals and groups, for nefarious activity.
Then again, it cannot be ruled out that the embassy personnel were up to something, if not exactly what Nicaragua charges. It is known that under William Casey the CIA has stepped up its activities throughout Central America. The covert aid given by the Reagan administration to the counterrevolutionaries or ''contras'' fighting the Sandinistas necessarily has fed suspicion in Managua and probably contributed to the spy charge.
Whatever the truth of the matter, the expulsions are regrettable, especially if they are used by the Nicaraguan government as an excuse to crack down further on its opponents. The incident is but the latest sign of a steady deterioration in Nicaraguan-US relations and that too is unfortunate. Yet Washington should take care not to overreact. It may, after all, have to live with the leftist Sandinistas for a long time to come, despite the armed rebellion against them. Many Western diplomats in fact argue that the Reagan administration has shortsightedly helped push Nicaragua toward Cuba and the Soviet Union by its patent dislike of the Sandinistas, by its failure to sympathize with the revolution, and by its economic aid cutoff.
That is arguable, perhaps. It is possible that fear and suspicion of the ''Yankee gringos'' is so great after decades of US neglect and misguided policy that nothing Washington did would have altered Nicaragua's Marxist-tending course. But in any case the US would do well to stay cool now and not cut off its bridges to Managua. Mexico and other countries are urging the US above all to engage Central America's leftists in genuine dialogue and help in a negotiated settlement of the conflict in the region. It is not yet clear that the administration believes that such dialogue can be effective.