There might be an option in readtable() or within the opts input that can specify what decimal character to use when reading in the data. I looked briefly but couldn't find what I'm imagining but I do recall that such an option exists. That would be the best solution, if possible.
To fix the data described in your question, first replace the commas in column 3 with periods.
C(:,3) = strrep(C(:,3),',','.');
then proceed with str2double.
T.Var3 = strrep(T.Var3,',','.');
T.Var3 = str2double(T.Var3);