The individual BF bond are polar but still BF3 is a nonpolar molecule .how?

The molecular geometry of BF3 is trigonal planar with symmetric charge distribution on the central atom. Therefore BF3 is nonpolar.

Or in less fancy words, the BF3 molecule has all its atoms in the same plane. The fluorine atoms are positioned at equal angles around the central boron atom (3 x 120° = 360°) so that all the charges cancel each other out.