However, the OPCW is not mandated to fix blame for the attack.
United Nations humanitarian chief Mark Lowcock described the pledges as a "good start" and said he expected the amount to grow.
The suspected poison gas attack in Douma on April 7 has sparked an ongoing clash of narratives between the West and the governments of Syria and its key ally, Russia.
Husari told The Associated Press on Wednesday that his production, which is still a work in progress, is a fictional short drama based on the chemical weapons attack in Ghouta that took place in 2013. The U.S., France and Britain also blamed the Syrian government, and launched punitive airstrikes a week after the attack.
On Tuesday, Russia's Kommersant newspaper, citing sources, said that S-300s would be given - not sold - to Syria and that issues surrounding the transfer "have practically been resolved". Their entry into the town was delayed by security fears.
Lowcock told reporters during an global donor conference in Brussels on Wednesday involving around 85 delegations that the figure was his "best guess" based on commitments made so far.
Russia's RIA Novosti news agency reported Wednesday that the Russian defense ministry would soon deliver modern air defense systems to the regime of Syrian President Bashar al-Assad, but did not specify that long-range S-300s would be part of the package.
Britain announced $630 million for 2018 and another $418 million for 2019, while Germany said it would donate $1.218 billion and the European Union pledged some $791.8 million.
A spokesperson for the Russian mission said the Russian and Syrian representatives would hold a news briefing for members of the organization on April 26 where they would speak about the alleged Douma incident. Britain vehemently denied the accusation.
Russian S-400 Triumph/SA-21 Growler medium-range and long-range surface-to-air missile systems are transported during the Victory Day parade at Red Square, in Moscow, on May 9, 2015.
Russia's Defense Ministry announced Wednesday that the planned delivery of an S-300 air defense system to Syria will be completed in the "near future", They did not provide an exact date, though Russian Federation is interested in shoring up Syria's air defense capability.
Russian Federation has arrayed S-300 and S-400 systems around its main bases in Syria near northeastern Latakia and has claimed that the U.S., France and Britain deliberately avoided the area for fear of interceptions during the April 13 missile strikes.
European Union diplomatic chief Federica Mogherini urged Moscow and Tehran, Assad's key supporters, to help bring him to the negotiating table, saying they had a duty to help wind down the war, now in its eighth year.