Formal Charge is calculated as, Lewis structure is attached below,
Formal Charge  =  [# of valence electrons] - [electrons in lone pairs + 1/2 the                                     number of bonding electrons]
# of valence electrons of Cl  = 7
electrons in lone pairs  =  4
number of bonding electrons  =  6
Formal Charge  =  [7] - [4 + 6/2]
Formal Charge  =  [7] - [4 + 3]
Formal Charge  =  [7] - [7]
Formal Charge  =  0
Result:           Formal charge on Cl in ClF₃ is 
zero.