It is even trickier than that

Russia and Iran support Syria's current leader al-Assad and they fight predominantly against rebels who tried to topple al-Assad. Russians and Iranians don't want to see any autonomous regions on the territory of Syria. Interestingly, Russians have been very passive with regard to |S|S.
Turks have trained and equipped some rebel factions (mostly ethnic Turkmens) with the aim of creating a buffer zone on Turkish-Syrian border. Turks don't want to see Kurds strengthening and setting up anything akin to a state on Syrian side of the border. Hence, Turks are at odds with the US.
The US, France, UK and few others have trained and equipped (1) Kurds to fight against |S|S, (2) Syrian rebels to press al-Assad to negotiate. The remnants of these rebels are now concentrated mostly in Idlib. The US and French presence remains strong in Kurdish areas and Kurds are a force to be reckoned with - as long as they get weapons and the US air cover keeps Russian air force at bay.
All in all, there is no unified effort to support any of the rebel or ethnic factions in the Syria. Turks and the US-led coalition have very different ideas about what needs to be done and particularly about Kurds.
At the same time, Russians and Iranians want to have in power a government that would support their aspirations that also differ. Russians would like to have a friendly state with Russian bases in it. Iranians want to have a pro-Shia state and a land bridge to Hezollah in Lebanon. The latter puts it in a direct confrontation with Israelis who have struck Iranian targets in Syria for over 200 times. Naturally, the US sides with Israel and has pressed Iran to leave Syria. This, in turn, has evoked strong Russian reactions.
Where do we stand now?
Idlib has become a home for all remaining rebel factions: Russians and Iranians battered them into submission in different enclaves and then offered an option to evacuate to Idlib. Rebels were happy to go and it effectively ended a spread-out insurgency that took enormous toll on Syrian armed forces. Now there's only one target area remaining for pro-government forces.
Kurds control a massive area in the north of Syria that they attempted to declare independent, but received no support or recognition. Syria's largest natural gas and oil reserves are in that area.
Turks are trying to expand areas at the border that are controlled by pro-Turkish rebels (Turkmens) and push Kurds eastward. This has brought them into confrontation with the US on several occasions. Simultaneously, Turks are posturing to keep Russians and Iranians away from the buffer areas they try to establish.
The most puzzling aspect of the situation are the US intentions. Kurds have served the US efforts to destroy |S|S extremely well and they bore the brunt of fighting and losses. Will the US simply leave Kurds to be torn to shreds by Turks and others? One must keep in mind that there are 40 million Kurds and if they try to establish their state, it would chop off territories from Turkey, Iraq and Iran. None of these states wants to see it. For Russians, it is a nice lever to turn these states against the US because of the US support for Kurds. Similarly, John Bolton, the new US NATSEC adviser to Trump has stated that Iran must be driven out of Syria. Is it serious?
So far, the only political support rebels in Idlib receive, is an outcry on the humanitarian grounds. It matters none to Russia, Iran and Syria. The US threatened to strike Syria if chemical weapons are used against rebels.
Turks are the only ones making forceful moves and putting their forces on the perimeter of Idlib, as well as supplying rebels with arms and ammunition, and it seems to hold Russians back. For now.