Why are Saudi Arabia, Qatar and Turkey overthrowing a sovereign regime ?
Not sovereign.
Assad had always a weak legitimacy in governing Syria.
People appeared to accept his rule (by force), and not by free and fair election. As long as Syria was stable no one gave a shit about "legitimacy"-questions.
Now, there's a situation where millions of Syrians are either dead or refugees, howitzers and aircraft involved in operations against cities, chemical-weapons.
Someone who is not democratically legitimized and who is one of the main actors in this mess (amount of killed people) can't have any claim to continue to govern the country. And he shouldn't be allowed to, it is either death or Den Haag.