No, no, no, a world war is not starting yet.

WWIII would involve a massive mobilization of superpowers, committing a giant % of industries and workers to manufacture ammunition and weapons.
Iran is a country of 90 million people, the 36th economy in the world with a GDP per capita 1/3 of Mexico's.
It is at best a regional conflict. Neither Russia, nor China, nor anyone came to their aid.

Wars are not won just with air and bombs (see the case of: Yemen, Vietnam, the Gulf) and who knows where this will end up.
But it is far from WWIII, we need to be more objective and not get carried away by the same old garbage propaganda from the media.