USB 3.0 Bandwidth Per Individual Port
The short answer from the USB spec, each port has a maximum throughput of 450 MB/s. But the USB 3.0 PCIe Host controller interface has a maximum throughput of 500 MB/S. USB 3.1 raise this to 1GB/s by changing the serial data from 5 Gbps to 10 Gbps and changing the data encoding. 

The 450 MB/s is the maximum data transfer rate for the whole USB 3.0 interface (It is a single lane PCIe interface). Just like USB 2 each port supports transactions that while underway take the whole bus at the exclusion of all other devices on all other ports for this Host controller.
If you had a 4 port card and had disk drives connected to each port, then the aggregate throughput for all drives combined cannot exceed the maximum of 450 MB/S (ignoring other protocol overheads) for the Host controller (the one lane PCIe interface). For each individual drive the throughput rate would be 450 MB/S for each data transaction, but an aggregate of 450/4 if all the drives were transferring equal data loads.