B is less electronegative than F. So B becomes the central atom.
If we have three F atoms, then we will be using all three electrons from the B. We divide the 360 degrees of a circle by the number of electrons, i.e. 3 and get the bond angle as 120 degrees. The VSEPR theory states that when the electron-pair geometry is trigonal planar the bond angle is around 118 degrees.
