Very, very unlikely. Iran is not going to start WW3, however, it could have a massive short to medium term rippling effect across the region and on the price of oil, thus raising prices across the board. This obviously is not good, even if it does not mean WW3.
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.