In this paper the stability of the differential intersystem biases (DISBs) for the observation values of overlapping frequencies is analyzed based on the observation values of the overlapping frequencies (BDS3 B1C/B2a, GPS L1/L5, Galileo E1/E5a) of BDS3, GPS and Galileo. On this basis, a tightly combined triplesystem realtime kinematic (RTK) positioning method considering DISBs is proposed. The ambiguity resolution (AR) performance and positioning accuracy for the conventional loosely combined model and tightly combined model are compared and analyzed with real test data. The analysis results show that for the baseline composed of the same type of receivers, the pseudorange and carrierphase DISBs among the observation values of the overlapping frequencies for the three systems are very close to zero and thus can be ignored. Compared with those of the conventional loosely combined model where the systems select their individual reference satellites, the model strength, the fixed rate and the fixed accuracy of singleepoch AR for the tightly combined model adopting common reference satellites are all improved to some extent, and the improvement is obvious especially for the obstructed environment with high cutoff elevation angle. In the aspect of positioning accuracy, the tightly combined model also has a certain improvement. In the condition of the cutoff elevation angle of 40°, the positioning accuracies in North/East/Up directions can be improved by 150%, 118% and 194%, respectively.