The eleven energetically lowest pure rotational transitions, J ←J−1 (J =1,2, . . . ,11), of H13CN, H13C15N, and HC15N in the ground and first excited bending state were measured. By operating the Cologne Tetrahertz Wave Spectrometer up to 1 THz in the sub-Doppler mode, a transition frequency accuracy of a few kHz is achieved. These measurements were carried out at frequencies between 80 - 950 GHz. In addition, some transitions of the three isotopomers with rotational quantum numbers J = 20,21,22,23 have been measured in Doppler-limited resolution near 2 THz, using the frequency stabilized Cologne Sideband Spectrometer for Terahertz Applications (COSSTA). Furthermore, direct l-type transitions of H13CN in the first excited bending state with J up to 35 have been measured. These new data are of particular importance, since we discovered highly excited circumstellar H12CN recently. A global fit of the newly enlarged data set together with existing carefully screened ro-vibrational data yields molecular constants which are highly reliable and of great importance both for astrophysical observations and laboratory applications