The BDS-3 preliminary system was declared functional on December 27, 2018. It enables positioning service on a global scale. This study conducts a comprehensive investigation of the BDS-3 global service from the perspective of positioning, including satellite ephemeris, benefits of the new BDS-3 satellites, and the new signal effect on three positioning modes: single-point positioning (SPP), precise-point positioning (PPP) and real-time kinematic (RTK). First, the broadcast and precise ephemeris availabilities of 18 BDS-3 Medium Earth orbit (MEO) satellites from January to June 2019 are investigated. Due to the shorter operational timespans of BDS-3 satellites, their broadcast and precise ephemeris files retrieved from Multi-GNSS Experiment (MGEX) have 1.2–19.4% and 10.9–13.77% discontinuities, respectively, while the discontinuities of BDS-2 satellite broadcast and precise ephemeris are only 0.1 and 2.1%. Second, the BDS-3 satellite orbit accuracies and clock precisions are significantly improved, i.e., below 0.5 m and 1.82 ns compared to 2 m and 2.91 ns for BDS-2 because of the inter-satellite links of BDS-3 satellites. Third, an average of 4.4–5.7 BDS satellites can be observed at stations located in North/South America, and 8.9 for the European station FFMJ located in Germany after the BDS-3 global system announcement. As a result, continuous positioning is feasible in these regions. As to four Asia–Pacific stations, their SPP accuracies are improved by 12.1–60.2% compared to those of the BDS-2-only solution. Meanwhile, the PPP and RTK convergence times of Asia–Pacific stations are also shortened. Last but not least, the new B1C signal does not bring convincing improvement to PPP and RTK positioning accuracies in this study, due to the limited number of available B1C new signals at this time.