Russia will enter with what?
Russia is already bogged down in Ukraine, they couldn't save Assad from a bunch of rebels on Toyotas, do you think they could take on Israel too?

There will be no WW3, nobody wants to defend Iran. Just like Palestine, behind politics, everyone in the Middle East hates them. So, who is going to help Iran? Saudi Arabia? Turkey? Pakistan? Nobody will move a muscle. Behind all their declarations to appease their Muslim population, they couldn't be happier to see the Iranian regime fall, everyone in the Middle East hates their proxy wars.

There will be no WW3, and if one starts years from now, it won't be because of Iran or Israel!