Why is BF3 triagonal planar and PF3 triagonal pyramidal?

Because, boron has 3 , and phosphor has 5 . If we are talking about molecule with 3 chlorine atoms then all of boron valence electrons are included in covalent bond, also 3 phosphor valence electrons are included in covalent bond. We can conclude that phosphor has 2 free electrons (one electron pair).

##BF_3##

##PF_3##