The starting point is conveniently working out with the "double bond equivalent" from the molecular formula. This shows the presence of one "double bond".

Two of the spectra (2 and 3) show presence of a broad peak, which is normally suggestive of N-H protons. Spectrum 3 also shows the characteristic ethyl multiplets, with the chemical shift of the CH2 group indicating it to be next to a C=O.

Spectrum 2 shows a CH3 single also at about 2 ppm attached to C=O and a rather strange doublet at about 3ppm. This is normally characteristic of N-CH protons, but why a doublet? We will return to that later!

The spectrum of 1 shows a peak at 8 ppm indicating H-C=O, but again two methyl singlets! These are unlikely to be couplings, since they have no partner (unless of course fluorine or phosphorus were present, but the formula indicates that they are not).

The strange properties of 1 and 2 are due to strong double bond character about the O=C-N bond (can you draw a resonance structure that illustrates this?), which in the case of 1 results in separate resonances fot cis and trans methyl groups.

For spectrum of 2, the explanation may be:

a) because there are two isomers presents

b) because of coupling between the N-H and the methyl group.

How would you distiguish between these two explanations?