References:
L.E. Kay, M. Ikura, R. Tschudin and A. Bax (1990) J. Magn. Reson. 89 496-514. (Link to Article)
S. Grzesiek and A. Bax (1992) J. Magn. Reson. 96 432-440. (Link to Article)
D.R. Muhandiram and L.E. Kay (1994) J. Magn. Reson., Ser. B 103 203-216. (Link to Article)

Minimum labelling: ¹⁵N, ¹³C

Dimensions: 3

Magnetisation is passed from ¹H to ¹⁵N and then selectively to the carbonyl ¹³C via the ¹⁵N^H–¹³CO J-coupling. Magnetisation is then passed back via ¹⁵N to ¹H for detection. The chemical shift is evolved on all three nuclei resulting in a three-dimensional spectrum.

This is the most sensitive triple-resonance experiment. In addition to the backbone CO-N-HN correlations, Asn and Gln side-chain correlations are also visible.
It is mainly used to obtain CO chemical shifts which can be used in a program like TALOS to help predict secondary structure.
The HNCO can also be useful for backbone assignment in conjunction with the HN(CA)CO, if the CBCANNH and CBCA(CO)NNH spectra are of bad quality.