"We also underline our concern at the potential for further - and illegal - use of chemical weapons," the statement said. "We remain resolved to act if the Assad regime uses chemical weapons again."
The three UN Security Council members released the joint statement to mark the fifth anniversary of the sarin attack in Ghouta that killed more than 300 people.
That attack, which the West blamed on Assad's forces, led to a US-Russian agreement to get rid of Syria's chemical stockpile and its capacity to produce deadly chemicals.
"Our position on the Assad regime's use of chemical weapons is unchanged," said the statement. "As we have demonstrated, we will respond appropriately to any further use of chemical weapons by the Syrian regime, which has had such devastating humanitarian consequences for the Syrian population."
In April the three countries launched airstrikes on Syrian government targets in response to an alleged chemical weapons attack in the town of Douma, which left dozens of people dead.
Speaking in Jerusalem on Wednesday, US President Donald Trump's national security adviser, John Bolton, warned that the US will responde "very strongly" if Assad's forces use chemical weapons in Idlib.
He also denied that Washington's sanctions are aiming to bring about regime change in Iran.
Assad's forces hold the south-eastern tip of Idlib and hopes to retake control of the whole province, which is the largest rebel-held area of Syria.The UN Security Council is to discuss the situation in Syria next week.