I don't have any problem with one RFID and Ethernet
How is the one RFID connected?
I saw that a few people used SoftwareSerial,
For a Uno that only has one hardware serial port, that is often needed. For a Mega that has 4 hardware serial ports, it is far less often needed.
Can someone help me?
Without seeing your code, a schematic, and knowing exactly what you need help with? Doesn't seem likely.