Your line width is 17cm: A4 paper is 21cm wide and you want margins 3cm wide on the left and 1cm wide on the right. With the shown input
you don't get an overfull box. Indeed the width of the table is
150mm + 6
\tabcolsep + 4*0.4pt = 163.215mm
\tabcolsep is usually 6pt and the vertical rules are 0.4pt wide. But your example is not what you show, since it' obvious that the second column is much wider than the first one (I suspect 100mm instead of 50). If the longtable preamble is
it's clear that the table will not fit in the line width. If you need a table where the second and third columns are twice as wide as the first one, you can do
Actually the first column is not exactly half of the second (it's shortened by 0.4pt), but this shouln't be a problem. This table will occupy exactly the line width. Just remember that LaTeX adds
\tabcolsep at both sides of each column.
You can play with this measures, as long as the sum of the factors to
\textwidth is 1, or try with Herbert's trick if the first column's width must be computed automatically.
Wide and long tables often require manual intervention.