documentclass{article}
setlength{parskip}{8pt}
begin{document}
This is the first paragraph

This is the second paragraph

This is the third paragraph
end{document}

{setlength{parskip}{0pt}This is the third paragraph}

documentclass{article}
setlength{parskip}{8pt}
begin{document}
This is the first paragraph

This is the second paragraphvspace{-8pt}

This is the third paragraph
end{document}