It is also helpful to post the actual error message.
As well, there are different validators available, keep a couple in favorites. All have message issues, I find that if I am not sure of the message in one... I will run it through another and some times make better sense of the error.
One possible issue could be the H3, although "legal", you can start a series with something like a H3, but can only follow it with H4, H5, H6. Headers should always start with H1 then H2 etc.
That said, there are times when using CSS, an H1 may be at the top in the final HTML, but in the code it is below another H Tag... this may fail a code validation though right in the end.
So as mentioned above it can be many things and the structure is right... but not enough to see the issue.