We just had some uphill battle with one of the boys who threw tantrum in the bus and even threw things on the floor. 
So how did we react to that?

Stay calm (actually ACT calm). Hold them close. Try not to have any eye contact with the passengers on board. Let the boy cry and scream all he wants.

Until-we-reach-home.

Put him at a corner. Start reprimanding. The boy cries. Leave him alone. Tell him what he had done was not acceptable. Ask him not to do it again (there will sure be many more times because he is only 3 years old).

Boy says sorry. End of a 1.5 hours battle.

Period.