I am assuming you mean grade 3 elementary school. This is the level I am responding for.
One or two attention getters -
eg Teacher: Look at me. (Eyes on me) or (Look at the teacher/Look at the tv) (choose one of these and use it)
Students: Look at you. (Eyes on you) or (Look at the teacher/Look at the tv.)
Teacher: Please be
Students : Quiet (with action of finger in front of mouth)
OR Teacher: 1, 2, 3
Students: Shhhh (and put finger in front of their mouth)
[again - choose one of these and use it)
Teacher: Put your hands on your
.. (head, book, pencil, shoulders
.)
Students (do the action)
I like to use actions a lot and during learning try to encourage students to use actions for the new words and phrases they are learning. When they are learning the phrases they can do the actions when they repeat after you. At other times you are modelling using the actions. [A physical response allows students to move and learn. Sometimes the actions trigger the words. There is the possibility that students will only learn the actions and not really hear what you are saying but I think the benefits outweigh this.)
Other phrases
Let's sing together. (I do the action of holding a microphone and then making a circular motion with one or both hands to encompass the students)
Listen and repeat (I do the action of cupping one hand behind my ear and then one finger touching my mouth and moving out - ie the direction of speech going out)
Hands up. [Hands down.] (I do the action of raising my hand as well as speaking)
(Everyone answer) (When I want everyone to call out the answer/their response at the same time I cup both hands behind both ears and slightly lean forward.)