My 7 year old who is in 2nd grade can perform in maths at much higher grade level than his. When his teacher is giving him 3rd grade math he is making several mistakes or not completing it. When I ask him he tells me he was thinking about something else or he does not know how he got it wrong. Any body else experience a similar issue?
That depends.
Is the third grade material much lower than your child's working level? If so, then I think that it is probably explained by boredom/lack of interest.
My daughter and I have looked at it this way:
a moped and a 12-cylinder sports car are BOTH fine methods of transportation, but one of them is perfect for a run down to the end of the street and back, and the other one... well, it seems a shame to even warm it up for such a task if that's all you plan to do.

Many HG kids have trouble being asked to do things that don't seem worth the effort of tuning in; sloppiness and procrastination rule the day when our DD11 is asked to do tasks like those. Her accuracy level increases dramatically when she is actually engaged.
We have observed that this is particularly true in math(s).
Try this-- make up about five problems for your child of two types. One type, make interesting APPLIED problems where he has to actually pull out the information and set up the problem hinmself. (Word/story problems) Then offer a few that are the plain variety, where it is merely a matter of going through the algorithm to do the operations.
My daughter ALWAYS did better on the former. Always. Her accuracy would be close to 100% on those, and entirely hit and miss on the latter. This, of course, is mostly true with material that is more than a year below her readiness level, but we see it periodically even with more challenging material. If it isn't :real: she simply doesn't care as much about the effort.
The problem isn't the student, and it isn't the math. It's often the manner in which the student connects (or does not) with the material.