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.