BF3 has zero dipole moment Consider the structure of NH3 as shown below.
In NH3,N is more electronegative than H . The direction of moment is same due to N-H bond as that of the lone pair of electrons on nitrogen. Consider the structure of NF3 as shown below, F atom is more electronegative than N, so all F atoms pull the electron towards themselves. Therefore, the resultant dipole moment is just opposite to the direction of the lone pair. Therefore, the order of the dipole moment is, NH3>NF3>BF3