Syria appears to be preparing the ground for a third Soviet SA5 antiaircraft missile site near the Jordanian border, and U.S. officials said yesterday that they see this as part of an expanding Soviet military presence in that strategically located Mideast nation.

The big question, said one official, is whether the Kremlin is sending its air defense specialists into Syria as temporary training teams or whether the plan is to try to establish a wider and permanent beachhead there, as the Soviets did in Egypt before then-President Anwar Sadat sent them packing.

The Soviets already have installed their modern, high-flying SA5 Gammon antiaircraft missiles at Homs, across from the northern tip of Lebanon, and at Dumayr, northeast of Damascus.

Now there are preliminary intelligence reports that they may be readying a third site for the SA5s at As Suwayda near the Jordanian border directly east of the Israeli Sea of Galilee.

The continued Soviet expansion of Syrian air defenses is likely to add one more complication to President Reagan's effort to negotiate the withdrawal of Syrian, Israeli and Palestine Liberation Organization forces from Lebanon, particularly if U.S. intelligence confirms beyond doubt that As Suwayda is to be the anchor of a north-south belt of air defenses.

SA5 missiles at southernmost As Suwayda, specialists said yesterday, would pose an especially worrisome threat not only to Israeli fighter aircraft but to the high-flying command posts Israel used so effectively in clearing the skies of Syrian aircraft during the invasion of Lebanon last year. The SA5 is credited with a straight-up range of about 95,000 feet and a slant range of up to 150 miles.

Syria is reported to be the first country outside the Soviet Union to have received the SA5, a 54-foot-long missile controlled from the ground by radio until it gets close enough to home in on its quarry with on-board radio. However, western specialists said the missile became operational about 15 years ago, suggesting it might be vulnerable to jamming and other electronic countermeasures.

The hopeful view among U.S. officials is that the Soviet Union wants to show Syria and the rest of the world that its weaponry, which proved embarrassingly ineffective in the air and on the ground against U.S. arms during the Israeli invasion of Lebanon last year, is indeed lethal if used properly. Under this scenario, the Soviets could leave the Syrians in charge of their own air defense once the SA5 training was completed.

The worst-case scenario at the Pentagon and the White House is that the Soviet overhaul of the Syrian air defense is just the thin edge of the wedge of the Soviet military presence in Syria and will make the Middle East even more volatile than it is now.

Israeli Foreign Minister Yitzhak Shamir on Jan. 4 said "we are troubled over reports" that the Soviets are manning Syrian antiaircraft defenses and other modern equipment. Israeli officials said then that Syria would be less willing to withdraw its troops from Lebanon once Syria has been rearmed. Israel has made Syrian withdrawal a condition for its own exit from Lebanon.

President Reagan yesterday had the harshest words yet for Israeli unwillingness to withdraw from Lebanon, declaring during a session with local television reporters at the White House:

"Israel is delaying, we believe, unnecessarily . . . . I think that there's a certain moral point that we think the Israelis are neglecting or not observing. And that is the new government of Lebanon, after all these years of revolution and upheaval, has aked all the foreign forces to leave. For them not to leave now puts them technically in the position of an occupying force, that they are there by force in this country that has said to them, 'We now want you to depart.' "