Lewis Acids are the chemical species which have empty orbitals and are able to accept electron pairs from Lewis bases. ${\mathrm{BF}}_3$ has $6$ electrons in the central metal atom, boron. Hence, it is electron deficient and is a Lewis acid. The other species ${\mathrm{OH}}^{-}$, ${\mathrm{NH}}_3$ and ${\mathrm{H}}_2\mathrm{O}$ are not having any empty orbitals in them, instead they can donate lone pairs(Lewis bases).