Simultaneously reading data from multiple serial ports
My question here is how can I simultaneously read out all the data from the different serial ports on the arduino mega sketch.


As you are using several serial ports data can already be received simultaneously. Each serial software class (Serial1, Serial2, Serial3) has an internal buffer where data is stored until it is read. 

You only need to read the data before the internal buffers are filled and there is a risk for over-flow. Check that data is available and read as usual per serial port. Avoid any blocking calls to avoid buffer over-flow. 

Cheers!