First, when you choose one of the three doors, you have only 33, (3)% chance of choosing the right one. So, probably, you chose the wrong door. Then, when the host opens one of the two doors that you didn't choose, to reveal it's the wrong door, the other one now have 66,(6)% chance to be the right door, while the door that you chose still remains with the 33,(3)% chance to be right. Therefore, you need to switch doors.
Shout by Soraia SilvaBlockedParentSpoilers2017-03-15T00:20:01Z
Kevin was right about the Monty Hall Problem.
First, when you choose one of the three doors, you have only 33, (3)% chance of choosing the right one. So, probably, you chose the wrong door. Then, when the host opens one of the two doors that you didn't choose, to reveal it's the wrong door, the other one now have 66,(6)% chance to be the right door, while the door that you chose still remains with the 33,(3)% chance to be right. Therefore, you need to switch doors.