ALL THREE of Western Europe's major governments have fallen into one kind or another of a midterm slump. Americans might usefully keep that in mind, since it is affecting the way Europe responds to the rest of the world. The French regional elections last weekend turned out badly for the governing Socialists and well for their conservative opponents. Since the campaign for next year's national parliamentary elections is already under way, the regional returns are another warning of trouble ahead for President Francois Mitterrand. His own term runs until 1988, opening the possibility that he may find himself trying to govern with parliament in the hands of the opposition. That's a familiar circumstance in American politics, but it has never happened in France's Fifth Republic, and the prospect creates great anxiety.

In West Germany earlier this month, Oskar Lafontaine, a radical Social Democrat, led his party to triumph in a state election in the Saar. That has no immediate effect on the conservative-to-center coalition in power at the federal level. But Mr. Lafontaine's victory gives new force to the neutralist attack on established German policy and significantly increases the strength of the left wing within the Social Democratic party.

In Britain, Margaret Thatcher's Conservative government remains securely in control with its large parliamentary majority, but it has been sinking recently in the public opinion polls. If national elections follow their usual rhythm, Britain, West Germany and Italy will all hold elections in 1987. Ruling parties in those three countries, unlike the French Socialists, are not yet close enough to the next vote to have to worry about it very urgently. But all of them are now halfway through their terms, with less to show for it than they had hope.

The general tendency in European politics is neither to the right nor to the left but, as it has been for some years, against the party in power. The widespread sense of discontent is generated above all else by the very high unemployment and the failure of a succession of experiments and initiatives to restrain its steady rise over the past decade. The interesting comparison between Britain and France suggests that the prescriptions of neither right nor left do much to cure the kind of unemployment from which both countries, and most of Western Europe, now suffer.

The causes lie deeper than conventional policy can reach. Most Europeans seem to be committed to social stability to a degree that is bad for economic growth. They know it, but they are sticking with their choice, however much they may complain about the side effects. The high unemployment hasn't led to political disruption or collapse, as politicians earlier feared. But it has made European politics a cheerless business in which governments currently find themselves grimly on the defensive.