Punishing an unruly 3 year old?

i have two four year olds and a five year old they used to act out in public so i started by talking to them i told them i didn't like the way they had acted and i wouldn't take them places if they acted that way again next time we went out sure enough they acted out so we left immediately all they way home they screamed and cried so when we got home the went straight to their room it only happened a few times after that but now the minute they act up i warn them once and we leave and the next trip they don't get to go as far as the screaming in the car on the way home i ask them once to quiet down then ignore them and they spend time in their room after they come out they have to tell you sorry