${\mathrm{BF}}_3$ is an acid, according to

${\mathrm{BF}}_3$ is a Lewis acid, as it accepts lone pair of electrons.

It is not an Arrhenius acid because it does not furnish ${\mathrm{H}}^{+}$ ion in its aqueous solution.

It is also not a Bronsted Lowry acid as if does not give ${\mathrm{H}}^{+}$ ions in any solvent.