Superpower Triangle Loses Its Hard Edges. DIPLOMATIC CALM

WASHINGTON is playing it cool and calm as the Soviets and Chinese move toward a summit meeting this year. Adopting a conspicuously relaxed posture, American officials indicate that the trend toward gradually warming relations between the two powers after years of strain is no cause for US concern.

``We shouldn't be blas'e,'' says a well-placed State Department official. ``We will watch to see if the Sino-Soviet rapprochement develops in a way that does not jeopardize our interests. If the expected cooperation leads to lower tensions, we and the world ought to benefit.''

This stance represents a significant change. There was a time when a blossoming rapprochement between China and the Soviet Union would have set teeth on edge in Washington.

In the past each power in the geopolitical ``triangle'' warily watched the other two and calculatedly maneuvered its bilateral relations with them to enhance its own geopolitical position.

Richard Nixon's opening to China in 1972 - just before a summit meeting with Leonid Brezhnev - dramatized the pressures that could be generated on Moscow to gain strategic advantage for the United States.

The ``watching'' still goes on. But the apprehension and manipulation in triangular politics appear to be fading, as each side of the triangle today enjoys improved relations with the other. With the growing thaw in Sino-Soviet relations, American officials say, the US position in the triangle is eroded, and it becomes more difficult for any side to manipulate the other two.

``Triangular diplomacy is not dead, but it has been drained of its tensions for the better part of the mid-'80s,'' a senior US official says. ``It has taken time, but especially with the emergence of Gorbachev and the acceleration of our relations with the Soviets, this has made the Chinese more interested in improving their own relations with the Soviet Union. So there's an element of change. With better arms control and a different aura around the Soviet Union, we don't need triangular politics.''

In Washington's view, all three players of the triangle benefit by the thaw between the two communist giants, whose relations deteriorated into a state of hostility, after a period of alliance in the '50s. US officials cite these common interests and benefits:

A reduction of regional tensions as the Soviet Union meets China's conditions for a normalization of relations: withdrawal from Afghanistan; end of its support for the Vietnamese occupation of Cambodia; and a drawdown of Soviet military forces on the Sino-Soviet frontier.

Continued discussions on resolving the problem of a divided Korea, although it is recognized that North Korea remains the stumbling block.

Eased tensions along the Sino-Soviet border and the start of an arms control dialogue that could lead to lower defense budgets.

A more stable global climate in which to concentrate on economic problems.

The change in the dynamic of triangular politics, diplomatic experts suggest, is due to the fact that the world is becoming ``multipolar.'' The two superpowers have declined in relative importance as other centers of power and influence - Japan, China, and India - have emerged and sought an independent role in world affairs.

``China is already close to the point where it's arguing that China must be included in international discussions affecting matters in the region,'' says Harry Harding, a China specialist at the Brookings Institution.

``Through arms sales it's also extending its sphere of diplomatic involvement to include the Middle East. It wants to be a major power that is involved in global issues,'' he says.

Economic factors are also seen to be driving the change in triangular diplomacy. Mikhail Gorbachev's accommodation with the US is dictated by a need to concentrate on internal reform. This is also propelling him to repair relations with China, which also wants to reduce tensions with Moscow and expand economic cooperation as it strives to modernize.

The various bilateral relationships now appear to be improving in tandem, without any of the three becoming alarmed. Because of d'etente with the Soviet Union, Washington is unconcerned about a Gorbachev meeting with Deng Xiaoping and other Chinese leaders. And China is less concerned about the Soviet-American accommodation because of its own improved relations with the US.

But diplomatic specialists caution that many of the changes are tactical and do not mean that triangular politics has become irrelevant.

China still views the Soviet Union and its ambitions in Asia as a strategic threat, and so does the United States. The Chinese have welcomed the US-Soviet d'etente, analysts say, but they are aware that they are losing leverage as a result.

``So the triangular metaphor is declining in salience but has not evaporated,'' Dr. Harding says.