The great thing about 3rd grade is that they still want to please the teacher, just like K. At the beginning of the year, I let the kids help me make the classroom rules. Honestly, they're already developed but you'll find that you can "lead" the kids into wording it the way you already have it.
I'll attach mine so you can see what we do.
You can say simple things like, "Thank you, John, for raising your hand before speaking". The others will follow! Praise goes a long way with 3rd graders!
I'm sure others will give you ideas, but that gives you a starting place. That's one thing I adore about 3rd grade - classroom management isn't that difficult.