In this paper, we study spectrum sharing of drone small cells (DSCs) network modeled by the 3-D Poisson point process. This paper also investigates an underlay spectrum sharing between the 3-D DSCs network and traditional cellular networks modeled by 2-D Poisson point processes. We take advantage of the tractability of the Poisson point process to derive the explicit expressions for the DSCs coverage probability and achievable throughput. To maximize the DSCs network throughput while satisfying the cellular network efficiency constraint, we find the optimal density of DSCs aerial base stations. Furthermore, we explore the scaling behavior of the optimal DSCs density with respect to the DSCs outage probability constraint under different heights of DSCs. Our analytical and numerical results show that the maximum throughput of the DSCs user increases almost linearly with the increase of the DSCs outage constraint. In order to protect the cellular user, the throughput of the DSCs user stops increasing when it meets the cellular network efficiency loss constraint. To further protect the cellular network in the spectrum underlay, we investigate the effect of primary exclusive regions (PERs) in a 3-D space. Unlike the circular PER in traditional cellular spectrum sharing in the 2-D space, the shape of the 3-D PER is found as a half sphere or a half sphere segment, depending on the radius of PER and the DSCs height limit. We show that the radius of PER should be restricted for small DSCs constraints and limited DSCs height.