documentclass{article}

usepackage{longtable}
usepackage{booktabs}
usepackage{enumitem}
usepackage{array} % Thanks daleif !

makeatletter
newcolumntype{P}[1]{>{@minipagetrue}p{#1}}
makeatother

setlist[itemize]{noitemsep, topsep=0pt, leftmargin=*}

begin{document}
 begin{longtable}{p{4cm}p{4cm}p{4cm}}
   toprule
    textbf{col1} & textbf{col2} & textbf{col3}  \
   midrule
    foo & bar & zbb \
    foo & multicolumn{2}{P{8cm}}{begin{itemize}item foo item barend{itemize}}\
   bottomrule
 end{longtable}
end{document}

documentclass{article}

usepackage{longtable}
usepackage{booktabs}
usepackage{enumitem}
usepackage{array} % Thanks daleif !

setlist[itemize]{noitemsep, topsep=0pt, leftmargin=*}

begin{document}
 begin{longtable}{p{4cm}p{4cm}p{4cm}}
   toprule
    textbf{col1} & textbf{col2} & textbf{col3}  \
   midrule
    foo & bar & zbb \
    foo & multicolumn{2}{p{8cm}}{
                                  begin{minipage}[t]{linewidth}
                                    begin{itemize}
                                      item foo
                                      item bar
                                    end{itemize}
                                  end{minipage}
                                 }\[10pt]
   bottomrule
 end{longtable}
end{document}