Dipole moment of compound having regular geometry and same type of atoms is zero. It is vector quantity.
The zero dipole moment of BF3 is due to its symmetrical (triangular planar) structure. The three fluorine atoms lie at the corners of an equilateral triangle with boron at the centre.
The vectorial addition for the dipole moment of the three bonds gives a net sum of zero because the resultant for any two dipole moments is equal and opposite to the third. The dipole moment of NH3 is 1.46 D indicating its unsymmetrical structure. The dipole moment of CH2CI2 (the molecule uses sp3 hybridization but is not symmetric) is 1.57D