From the SoftwareSerial reference page
The library has the following known limitations:
If using multiple software serial ports, only one can receive data at a time.
and
If your project requires simultaneous data flows, see Paul Stoffregen's AltSoftSerial library. AltSoftSerial overcomes a number of other issues with the core SoftwareSerial, but has it's own limitations. Refer to the AltSoftSerial site for more information.
You can also manage 2 or more soft serial ports using the listen() method but it is not exactly convenient.