Contact your administration if you feel a particular standard needs to be changed. Keep in mind that 3rd grade standards, as with the rest of the learning and benchmark standards, are often set by your state or even the national government. If this is the case, it will take many steps to try to improve them. However, if your school or district has set standards, you can talk to your administration to get the ball rolling for those particular standards.
Explain why you feel the local or district standards need improvement. Some reasons might be that your 3rd grade students aren't capable of learning the material, or that the material is at a lower level, instead of being at 3rd grade material. You might also reason that the standards conflict with social norms or religious ideas.
Meet with the parties responsible for setting the district or local standards. Usually, this is the school board. You'll be better off if you can find other 3rd grade teachers who agree with you. Bring many ideas to the table when it comes to how to improve the standards - adding more material, taking away specific items, or changing particular standards. Keep in mind that while you don't have decision-making power, you can influence those who can enact change.