Since the ratio of the angle measures is $ 3:2:1 $, the angles have measures $ 3x $, $ 2x $, and $ x $ for some value of $ x $. The angle measures of a triangle sum to $ 180^\circ $, so we have $ 3x+2x+x = 180^\circ $. Simplifying gives $ 6x=180^\circ $, so $ x=30^\circ $. Therefore, the angle measures are $ 90^\circ $, $ 60^\circ $, and $ 30^\circ $, which means the triangle is a 30-60-90 triangle. In such a triangle, the hypotenuse has twice the length of the leg opposite the $ 30^\circ $ angle, so the hypotenuse of the triangle in the problem has length $ 2\cdot 12 = \boxed{24} $ meters.