Microcontroller with more than one serial port
There are multi-drop serial interface standards, e.g. RS-485. You could investigate if all these parts can be configured to support such a standard, and if you can arrange a suitable protocol to avoid everybody talking at once.
Or you could use external gates or switches (e.g.74HC4052) to route your serial signals to one device or the other, controlled by a port pin from the processor.

This doesn't work if both RFID reader and WiFi need to be able to interrupt the CPU at random times, though you could add other connections to allow that.