Three year olds are experiencing school for the first time. USe positive reinforcement rather than threats or getting angry or frustrated. Patience is essential.

Rather than use your voice by getting louder than the children, have a nonverbal signal: lights flashing, finger over lips to cue silence, put arm up. Reinforce the behavior you want to see as each child quiets down, "I see John is quiet and sitting down. Thank you, John." Keep going until everyone is settled. This will take time at first, but serve as a foundation for the children to understand that they need to be quiet and listen. This may also be connected to a reward such as being first in line, being the leader of the group, etc. You may also want to use a stuffed animal for the person who has permission to talk. Then they learn that the person who holds the stuffed animal may speak; the others listen by looking at the person to understand what they are saying.