Terrible Two or Three???

Why is it called the Terrible two's when Three years olds are so much worse. Bo was a great 2 year old mind you he didn't talk much. but now that hes turned 3 he has kept being a angel to everyone else but when we are home and no-one else is around he is a monster, yelling at me, not listening to anything i say, and beating up and bossing his sister around. not sure if i can last a year of this