Two of the Syrians were questioned at UN offices in Vienna on Monday and returned to Damascus. The three others were questioned on Wednesday and are expected to return home immediately.
"It's finished for the whole session ... I hope there will be nothing more until 15 December," said a Syrian diplomat who declined to be named.
The UN commission investigating al-Hariri's death is expected to hand in its final report by 15 December.
Unnamed diplomats quoted by the Austrian Press Agency said that the witnesses included Abdelkarim Abbas, the head of the Palestinian affairs department, and communications chief Zaher Yussef.
A truck bomb blast killed al-Hariri
in Beirut on 14 February
According to unconfirmed reports, the "third man" at the hearings was probably Hussam Taher Hussam, a Syrian living in Lebanon seen as a key figure in the UN investigation who on 28 November said he was forced to testify under duress to the commission led by German prosecutor Detlev Mehlis.
Syria, which denies any role in the killing in February, agreed to let international investigators question the men at UN offices in Vienna after winning guarantees from permanent UN Security Council member Russia that they would be allowed to return to Damascus.
An interim report in October by Mehlis, suggested the 14 February bombing that killed al-Hariri and 22 other people was planned by top Syrian security officials in Damascus and their Lebanese allies.
On Mehlis's recommendation, Lebanon has charged four pro-Syrian Lebanese generals with murder.
The UN probe team has so far quizzed more than 500 people and Mehlis says his team might ask for more Syrian officials to be interviewed.
Al-Hariri's killing triggered mass anti-Syrian protests in Lebanon, forcing Damascus to pull its troops out of its small neighbour after a 29-year presence.
Syria had sent forces into Lebanon in 1976 to quell a civil war.