No need to change the Heading3 paragraphs. Look at the reference page of the table of contents. There is probably a page tagged TOC that defines the format of the entries in the TOC. It has paragraphs whose tags are derived from the paragraphs in the main document that generate entries by appending TOC to the original tag. Thus, entries for Heading3 paragraphs are tagged Heading3TOC. The reference page has a Heading3TOC paragraph that looks something lke:
<$paranumonly> <$paratext>.........................<$pagenum>
where there is a tab character between <$paranumonly> and <$paratext> which moves the left side of the <$paratext> to 2.0cm and another tab between <$paratext> and <$pagenum> which moves the right side of the <$pagenum> to 12.0cm.
I am suggesting putting a space before the second tab. You might also add an extra tab before the <$pagenum>.
--Lynne