We need to solve various mobility/traffic problems in one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) micro or picocell environments to efficiently plan future personal communications services (PCS's), However, mobility/traffic problems have thus far primarily been studied in 1-D and 2-D cell structures. In this paper, we extend the previous mobility modeling from 1-D or 2-D space to 3-D unbounded indoor building environments having staircase regions by analytically modeling mobility to estimate the number of handoffs, We also characterize the blocking probability of each cell according to mobility by predicting the equivalent input traffic per cell. Based on the blocking probability model, we obtain the required number of channels per cell under the given blocking probability constraint, For example, 13 channels per cell are required to be assigned in order to meet the requirement that the blocking probability does not exceed 0.02 in the case where the number of radio ports (RP's) (cells) is four and 180 users are moving with the mean 2 km/h (horizontal motion) and 2/3 km/h (vertical motion) on each floor. The computer simulation results are also close to the analytical ones. These results can be utilized in the network planning of future PCS's.