Compound (1) has an asymmetric nitrogen atom, while compound (2) does not. Similarly, compound (3) contains an asymmetric nitrogen atom, but due to the inversion of the lone pair of electrons, the isomers rapidly interconvert, and it exists as a single compound.

Compound (1) has an asymmetric nitrogen atom, while compounds (2) and (3) do not have enantiomer pairs because they are meso compounds.

Compound (1) has an asymmetric nitrogen atom, while compounds (2) and (3) do not have enantiomer pairs due to the planar structural arrangement.

Compound (1) has an symmetric nitrogen atom, while compounds (2) and (3) do not have enantiomer pairs due to presence of the asymmetric nitrogen atoms.