THE TRIUMPH of Roy Jenkins and Britain's

new Social Democratic Party is the most recent of three European elections conveying similar messages. Over the past two weeks there have been the French local elections, a German state election in Lower Saxony and now a special election in Glasgow to fill a parliamentary seat. While there is no consistent movement to the right or the left, each of the three was a vote against the party in power.

But whatever exasperation European voters may feel against their governments, it is also worth noting that there was no inclination whatever to move outside the familiar limits of conventional politics. In France, the biggest loser was not President Mitterrand's Socialist party but its coalition partner, the Communists. Lower Saxony gave no encouragement to the far left--the opposition to Chancellor Schmidt that has sprung up within his own Socialist party. Instead, former Socialist voters turned to the small environmentalist party or stayed home.

This tendency toward the center is even clearer in the British case, and particularly important to Britain's allies. Mr. Jenkins won in a constituency that has been consistently Conservative for two generations. A lot of voters there are clearly fed up with Mrs. Thatcher and the prolonged recession that her economic program has produced. But the traditional opposition, the Labor Party, is drifting further and further into an unappealing mixture of ideological rigidity and sentimental isolationism. Thursday's returns give plausibility to a third party on a path that moves sanely between Mrs. Thatcher's monetarism and a left that stands for no nukes, no Atlantic alliance and no Common Market. Gratification, not to say relief, will not be confined to Britain.

A postscript: although the foreign news is, reasonably enough, reported mainly in terms of foreign policy, it's useful for Americans to keep it in mind that European politics is deeply preoccupied with domestic economies. As in this country, elections there are being won and lost primarily on such issues as inflation, jobs, social security and public deficits. One consequence is that for the present, none of the European governments is in a position to provide strong leadership in those matters of foreign and strategic policy about which the Reagan administration cares most.