Maybe I can help someone as I found the problem.
The problem was actually with the Sygil "heading_id#" tag, so my H3 title was tagged <H3 heading_id#>TITLE</H3>. To quicky make an H3 title I've cut and pasted the first H3 TITLE for all the H3 title in the xhtml file. So every H3 title was tagged with the same heading_id# creating a problem for Sygil in the TOC creation. Every H3 title was so linked with the first H3 title.
Thank you all by the way!!!