Using multiple UARTs

I am planning to use Arduino Due to use its four UART(during testing i will be using two UART). What i want to do is to take some commands from OBC using Serial3 (TX3 & RX3).And store those data in the memory for a defined period.Then read those commands using Serial 0 (Programming port) in the terminal(i am using tera term).But i am not sure how to approach this.
My difficulty arise from the fact that i have no idea how and where the data from OBC will stored in arduino.And after storage how to access those data via terminal.

Any suggestion will be helpful.

Thanks,
Bikash

i have no idea how and where the data from OBC will stored in arduino

The how is because you read the Serial port that the data arrives on.
The where is a deep dark mystery that YOU have to answer.