This paper presents the findings of an extensive measurement study on multiple commercial 3G UMTS networks. We have investigated the performances of those 3G networks in terms of their data throughput, latency, video and voice calls handling capacities, and their ability to provide service guarantees to different traffic classes under various loading conditions. Our findings indicate the diverse nature of network resources allocation and call admission control policies employed by different operators. It is also found that the 3G network operators seem to have extensively customized their network configurations in a cell-by-cell manner according to the individual site's local demographics, projected traffic demand and the target coverage area of the cell. As such, the cell capacity varies widely not only across different operators but also across different measurement sites of the same operator. Even for the same site, the capacity can easily change by more than 10% across multiple measurements taken at different time of the day. The results also show that it is practically impossible to predict the actual capacity of a cell based on known theoretical models and standard parameters, even when supplemented by key field measurements such as the received signal-to-noise ratio (Ec/N0).