The tendency of ${\text{BF}}_3,{\text{BCl}}_3$ and ${\text{BBr}}_3$ to behave as Lewis acid decreases in the sequence:

The Lewis acidity of boron halides is governed by $p\pi -p\pi$ back-bonding. In ${\text{BF}}_3$, the size of $2p$ orbitals of B and F match perfectly, allowing strong back-donation of electrons from F to B. This reduces the electron deficiency of B, making ${\text{BF}}_3$ a weak Lewis acid. As the halogen size increases ($\text{Cl}\to \text{Br}$), the orbital overlap becomes poorer, back-bonding weakens, and the Lewis acidity increases.Order: ${\text{BBr}}_3>{\text{BCl}}_3>{\text{BF}}_3$.