Just use the string input option with the terminator set to 13.
Code:
MyVar VAR BYTE(3)

Main:
    HSERIN [STR MyVar\3\13] ' receive up to 3 characters, terminate input on receipt of 13
    HSEROUT [STR MyVar\3,13,10] ' show it
    PAUSE 100
    goto Main
This will receive up to 3 bytes, 0-160, and terminate reception on receipt of 13.