Maybe Iran will not start the WW3, but if USA keeps pushing them they will not have other options than to attack. If Rusia or China decide to protect Iran then it could be the start of WW3. The odd of this scenario are really low, but is possible.
Anyway, if it happens a lot of people will die, but it will not be the end of the world.
The chances of having WW3 is very low now as of the moment. It seems that the tension between the United States and Iran have been dropped which is a good thing. And the probable cause of that shrinking tension is the fall of an airplane of Ukraine because of Iran's failure in judgment.
I think there's this quiet tension among the concerned countries especially because iran admitted shooting down a passenger plane. Justin trudeau already said they won't take this sitting down unless justice is served as there are several canadians onboard, as well as ukranians. The us hasn't really said much about that at this point but on small spark could escalate things real quick.