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Since December 27, 2012, BDS-2 has provided navigation, positioning and timing (PNT) services in the Asia Pacific region. From November 2017 to the end of August 2018, the BeiDou Global System (BDS-3) has successfully launched 12 satellites. With the development of BDS, time transfer based on BDS has become a hot topic. In this work, 3 tracking stations, BRCH (PTB), XIA5 (NTSC) and WUH1, of the International GNSS Monitoring and Assessment System (IGMAS), 31 consecutive days of observation data, are selected. The real-time time transfer based on BDS-2, BDS-3, BDS-2+BDS-3 three schemes are preliminarily analyzed. After that, compared with the results based on GPS precise point positioning (PPP) method, the preliminary results shown that the standard deviation (STD) value of BRCH-XIA5 link based on BDS-2 is 25.1 ns, and the STD value of BDS-2+BDS-3 combination is 20.5 ns, which is increased by 18.33% in the case of unfixed station coordinates. The STD value of WUH1-XIA5 link with BDS-2, BDS-2+BDS-3 combination is less than 7 ns, and the improvement degree of BDS-2+BDS-3 is relatively small, which is about 1.47%. For frequency stability, the stability of the BRCH-XIA5 link BDS-2+BDS-3 combination is better than that of BDS-2, while WUH1-XIA5 link show a similar level with two schemes. In the case of fixed station coordinates, compared with the STD values of BRCH-XIA5 link with BDS-2 and BDS-3, BDS-2+BDS-3 combination is increased by 23.08% and 45.01% respectively. The values of the WUH1-XIA5 link with BDS-2 and BDS-2+BDS-3 combinations are all less than 1.5 ns. The STD of BDS-2+BDS-3 is reduced approximately about 7.14%, as compared to BDS-2; the STD value of BDS-3 scheme is 3.2ns, which is slightly larger than BDS-2 and BDS-2+BDS-3, due to the fact that BDS-3 satellites are less observed at this stage. For frequency stability, the stability of two link with BDS-2 and BDS-2+BDS-3 combination is basically equal, while BDS-3 is slightly worse. In addition, and the stability of WUH1-XIA5 link clock offset is better than that of BRCH-XIA5 link.