You might be tempted to reward your third grade class with food. There are so many options: candies for following the rules, a class ice cream party for group good behavior, maybe tickets as rewards that can be turned in for brownies or some other treat. Think twice, though, before going this route. One or more of your children may have an allergy that you don't know about. Food rewards can also lead to unhealthy relationships with food later in life as children learn to reward behavior with treats.
If a child forgets his coat in the morning and the temperature drops in the afternoon, he'll have to spend recess indoors. If another child brings his coat, even though the morning is warm, he'll be prepared for outdoor recess even if it's cold. These are natural consequences. They don't require much from the teacher, other than perhaps facilitating the consequence. These natural consequences are easy for students to tie to their own choices and behaviors, and so often lead to students naturally making choices that avoid negative consequences and achieve positive consequences.
Verbal praise is a simple but effective form of reward. Make sure to catch students behaving in a positive manner. Tangible items, such as tickets or tokens are another way of rewarding positive behavior. For instance, you could put a marble in a jar when students are behaving especially well, and then reward the class when the jar is filled. Students can also collect individual tickets or tokens to turn in for larger rewards such as getting homework passes, being first in line, choosing their own classroom jobs or receiving recognition in the class newsletter.
The first consequence for breaking a rule or exhibiting negative choices or behavior might be a verbal warning. Often this is enough to put the student on the right path to better choices. If the behavior continues, a non-verbal warning, such as writing the child's name on the board might be necessary. For more serious behavior problems, the student may need to lose points or tokens that he'd previously earned, or eventually a visit with the principal or a call home. For consequences to be effective, have a clear list of rules and consequences that students understand from the beginning.