Signal-in-Space Monitoring Accuracy (SISMA) is regarded as a key indicator for the integrity monitoring of the BeiDou Navigation Satellite System (BDS-3). It is used to evaluate the quality of broadcast ephemeris in real time. However, the conventional SISMA estimation method in the BDS-3 ground operation and control system is severely constrained by the deployment of ground monitoring stations, which are mainly located within China. As a result, the monitoring arcs for BDS-3 in regions outside China are incomplete and exhibit considerable latency. To overcome these limitations, this paper proposes an integrated space–ground SISMA monitoring method. Observations from Low Earth Orbit (LEO) spaceborne receivers are combined with those from ground monitoring stations. A globally distributed space-based monitoring network is constructed using 26 LEO satellites (18 near-polar, 8 inclined-orbit). These satellites act as dynamic extension nodes, while the ground network provides the backbone reference. This integrated network fuses the two types of observations through a weighted strategy, where the weights are determined by a priori observation precision of each data source. The proposed method is validated using measured data from 6 November 2025. Experimental results show that, compared with the conventional BDS-3 SISMA monitoring approach, the proposed method reduces the overall unmonitored proportion from 71.62% to below 3%. The monitoring arc coverage for Medium Earth Orbit (MEO) satellites increases from 21.12% to 97.43%, and that for IGSO satellites rises from 64.39% to 100.00%. These results confirm that the integrated space–ground approach effectively enables global SISMA monitoring for BDS-3. The integrated space–ground approach effectively fills monitoring gaps in oceanic, polar, and other areas outside China, achieving continuous and stable global SISMA monitoring for BDS-3. This advancement can strongly support high-integrity services such as civil aviation and autonomous driving.