A new technique was used for deriving spectroscopic constants by reference to the known constants of a related electronic state. Applications of this method to several diatomic molecules revealed its usefulness and encouraged its use with the triatomic hydrogen molecules. Spectroscopic B0 constants for Rydberg states of H3 and D3 are obtained. Predicted and observed B0 values for the upper state of the 3d(A1′)→A2″(2p) band are 21.87 and 21.96 cm−1, respectively.