Arduino mega-serial communication issues

While interfacing the three modules i,e. Biometric Sensor, RFID and Keypad with Arduino Mega, these do not work simultaneously. Only the Biometric sensor module works properly when the codes of all three modules are combined . But when I interface two modules i,e. RFID and keypad, they work properly even display the proper messages on serial monitor. The modules also work properly when interfaced individually, but the issue only rises when all three are interfaced at a time. kindly help me out . Thanks

My crystal ball is broken.

You're going to have to supply more information, preferably in code tags.

Mine too.

...R