What is the relationship between the Syrian government forces, Syrian anti-government forces, and IS?

Hostile relations

The struggle within Syria is mainly between government forces and anti-government forces, but it is also mixed with IS armed forces. The three formed a Syrian version of the "Romance of the Three Kingdoms". The government forces are the Allawi branch of the Shiite Muslim community represented by its legal president Bashar al-Assad. The anti-government armed forces are an alliance of armed forces from multiple factions and basically represent the forces of the Muslim Sunni sect. The contradiction between the two is almost irreconcilable. The so-called "Islamic State" (IS) is an extremist terrorist organization. It is a product of the United States' anti-terrorism and intervention in Middle East affairs. It is precisely because of the struggle between the Syrian government and the rebels that it has been given the opportunity to take advantage of the situation. On the surface, neither the Syrian government nor the anti-government forces want to have contact with IS, but in fact they both hope to use the organization's power to consume and contain the other party's strength.