\footnotetext numbering for many \footnotemark - automatic solution

\documentclass{report}

\begin{document}

Text\footnote{text1} Text\footnote{text2}

\begin{itemize} 
 \item X\footnotemark{} \item Y\footnotemark{} \item Z\footnotemark{} 
\end{itemize}

 \addtocounter{footnote}{-3} %3=n
 \stepcounter{footnote}\footnotetext{a}
 \stepcounter{footnote}\footnotetext{b}
 \stepcounter{footnote}\footnotetext{c}
\end{document}

An alternative would be to fix the environment to not need this, and lift the footnotes out of the environment so manual correction isn't needed. tabularx for example does

 \let\@footnotetext\TX@ftntext\let\@xfootnotenext\TX@xftntext

at the start of the environment and

\global\TX@ftn\expandafter{\expandafter}\the\TX@ftn

at the end, where \TX@ftntext is

% \begin{macro}{\TX@ftntext}
% \begin{macro}{\TX@xftntext}
% Inside the alignment just save up the footnote text in a token
% register.
%    \begin{macrocode}
\long\def\TX@ftntext#1{%
  \edef\@tempa{\the\TX@ftn\noexpand\footnotetext
                    [\the\csname c@\@mpfn\endcsname]}%
  \global\TX@ftn\expandafter{\@tempa{#1}}}%
\long\def\TX@xftntext[#1]#2{%
  \global\TX@ftn\expandafter{\the\TX@ftn\footnotetext[#1]{#2}}}
%    \end{macrocode}
% \end{macro}
% \end{macro}

So putting it all into your example:

\documentclass{report}
\makeatletter
\newtoks\FTN@ftn
\def\pushftn{%
 \let\@footnotetext\FTN@ftntext\let\@xfootnotenext\FTN@xftntext
  \let\@xfootnote\FTN@xfootnote}
\def\popftn{%
 \global\FTN@ftn\expandafter{\expandafter}\the\FTN@ftn}
\long\def\FTN@ftntext#1{%
  \edef\@tempa{\the\FTN@ftn\noexpand\footnotetext
                    [\the\csname c@\@mpfn\endcsname]}%
  \global\FTN@ftn\expandafter{\@tempa{#1}}}%
\long\def\FTN@xftntext[#1]#2{%
  \global\FTN@ftn\expandafter{\the\FTN@ftn\footnotetext[#1]{#2}}}
\def\FTN@xfootnote[#1]{%
   \begingroup
     \csname c@\@mpfn\endcsname #1\relax
     \unrestored@protected@xdef\@thefnmark{\thempfn}%
   \endgroup
   \@footnotemark\FTN@xftntext[#1]}

\makeatother

\begin{document}
Text\footnote{text1} Text\footnote{text2}


fff\footnote[35]{jjjj}

\vbox{\pushftn
\begin{itemize}
\item X\footnote{a}
\item Y\footnote{b}
\item Z\footnote{c}
\item W fff\footnote[42]{kkk}
\end{itemize}
}\popftn


\end{document}

For tables the tablefootnote package implements the automation asked for (and also takes care of hyperlinks, when the hyperref package is used). (Please see its manual.pdf.) For other environments, the package can be "abused" (see explanation at my answer to "Nested footnotes disrupts comma delineation of footnotes"). It saves the (table)footnote(text)s and issues it/them after the end of the table environment. When you use a vbox, it must be (manually) issued at its end, which \spewfootnotes does. Complete automation would redefine \vbox to include \spewfootnotes after the end of the box and redefine \footnote to be a \tablefootnote inside of a \vbox.

\documentclass{report}

\usepackage{hyperref}% if you want/for demonstration of  hyperlinks
\usepackage{tablefootnote}

\makeatletter
\newcommand{\spewfootnotes}{%
\tfn@tablefootnoteprintout%
\global\let\tfn@tablefootnoteprintout\relax%
\gdef\tfn@fnt{0}%
}
\makeatother

\begin{document}
Text\footnote{text1} Text\footnote{text2}

\vbox{%
\begin{itemize}
\item X\tablefootnote{a}
\item Y\tablefootnote{b}
\item Z\tablefootnote{c}
\end{itemize}
}\spewfootnotes

Text\footnote{text7}

\newpage

Just to get another page to demonstrate the hyperlinks.

\end{document}