How to expand row height while using long text in multirow?

How to expand row height while using long text in multirow?

I m working on one table which has one row which spans over 3 rows but
text in this row is very lengthy so it uses more than 3 lines so all text
is not visible. How can i increase row height dynamically to fit this row.
\documentclass[crop]{standalone}
\usepackage{multirow}
\begin{document}
\Huge
\begin{tabular}{|c|c|c|}
\hline
\multirow{3}{*}{\parbox{8cm}{This is line1 ~\\ ~\\ This is line 2 ~\\
~\\ This is line3 ~\\ ~\\ This is line 4}} & cell1 & cell2 \\
\cline{2-3}
& cell3 & cell4 \\ \cline{2-3}
& cell5 & cell6 \\ \hline
\end{tabular}
\end{document}
line3 and line4 are not visible as it uses more than 3 row space.
As shown in the image:

How can i control it?
Regards