Users in a hotspot area may experience low Quality of Service (QoS) owing to the high traffic demands and limited bandwidth resources in downloading traffic from the nearby Macro Base Station (MBS). Deploying Drone Base Stations (DBSs) over a hotspot area to help the MBS transmit traffic to the users in the hotspot area is an efficient way to improve the user QoS. Essentially, DBSs are considered as relay nodes, which are deployed close to users, to transmit traffic to users with high data rates. This paper discusses the multiple DBSs 3D deployment strategy in a given hotspot area, i.e., determining the latitude, longitude, and altitude of each deployed DBS such that the number of the deployed DBSs is minimized while QoS requirements (in terms of pathloss requirements) of all the users in the hotspot area are satisfied. The strategy is formulated as an optimization problem, and a novel heuristic algorithm, i.e., Adaptive MUltiple drone base Station placEment (AMUSE), is designed to efficiently solve the problem. The performance of AMUSE is demonstrated via extensive simulations.