Trying to motivate kids can be difficult sometimes, whether they are at home or at school.  It sounds as if your son is capable of doing the work when he is able to focus.  Your son may be just trying to adjust to the different work load and expectations of 3rd Grade.  This adjustment takes time.  As a parent, it is important to be patient sometimes with our kids and teach to them the importance of reading, writing and completing homework.  
 
You mention that he is not responding to you when you are trying to talk to him.  Do you have him give you eye contact when you talk to him?  This will enable him to concentrate on your instructions. Have you issued any consequences for this behavior?  Try and make it clear to him that if he chooses to not respond, he will loose some of his privileges.  Examples are: loosing T.V. time, computer games, video games, playing with friends for the day or loosing something that is meaningful to him.  Remember that consequences change behavior.  And should he choose to not listen to you, not complete his homework he will continue to have limited or no time for his privileges.  You want to motivate him to listen to you, and do his homework and if he chooses to complete his homework and listens to you, then he will have access to his privileges.
 
Some other questions to ask yourself as a parent:  Is my son getting enough sleep?  Is he eating the right foods and hydrated?  Are there too many distractions at home during homework time?  Is there a set time for homework to be completed?  Is he allowed short breaks during homework time?  Can you talk with the teacher about your concerns that you are observing at home to make sure he is not acting this way at school?  Can you make the reading, writing and homework fun for him?  Have you sat down and talked with your son about his feelings of his school work and homework?  Is it too difficult for him?
 
By teaching your son how to make good choices at home you can motivate him to do well.  Remember to keep the consequences age appropriate and he is able to earn some privileges back based on good choices and decisions.  Keep praising him when he is doing good, tell him you are proud of him and give him a hug!

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