In BF3, B is sp2 hybridised and has one vacant p-orbital which gets filled by accepting a lone pair of electron present on the N-atom of NH3. Nitrogen in this adduct acts as donor atom and BF3 acts as an acceptor. So, hybridization of B in BF3 changes from sp2 to sp3 whereas there is no change in hybridization of N in NH3 and in the adduct.