Avoid an automatic new page when inputting a section after a longtable

When I start a new section after a longtable, I find the section begins at a new page and leaves a blank page between the longtable and the new section. But when I try just input some usual text, it just prints just at the end of the table. I want the next section following the longtable.

I find the similar answer in the here, but when I want to change the size of the following:

begin{longtable}
{p{3cm}<{centering}p{8cm}<{centering}p{3.5cm}<{centering}}

I do not know how to do exactly. Maybe I do not understand the theory or reason of this method. Could you provide a method or explain why the above method can be used? Thanks in advance!