Low Earth orbit (LEO) satellite communication in the sixth-generation (6G) of cellular networks is expected to redefine the fabric of global connectivity through the integration with aerial platforms. However, for the successful deployment of the LEO satellites, the excessive Doppler shift issue should be addressed by effective estimation and compensation techniques, as they move rapidly through their orbits. Therefore, in this letter, we investigate such Doppler shifts of the links between the satellite and an unmanned aerial vehicle (UAV) swarm in a three-dimensional space. We derive an approximate probability distribution of the Doppler as a function of the satellite orbit, the UAV swarm location, and the UAV cluster size. Moreover, we present the differential Doppler (DD) caused by the random dispersion of the UAVs. Through both simulation and analysis, we observe that the DD increases as the elevation angle and cluster size increase. In particular, when the radius of the UAV cluster is larger than 4km, the DD may not be supported by 3GPP standard. Therefore, our analysis provides design insights to determine the key system parameters of the UAV swarm to keep the DD below the system requirement.