documentclass[a4paper,12pt]{article}

begin{document}

begin{tabular}{|p{1cm}|p{2cm}|p{3cm}|}
    hline
    1 cm  & 2 cm  & 3 cm  \
    hline
end{tabular}

Table with actual widths

begin{tabular}{|p{0.56cm}|p{1.56cm}|p{2.56cm}|}
    hline
    1 cm  & 2 cm  & 3 cm  \
    hline
end{tabular}

end{document}