Here is the alphabet that you have learnt about.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
I have made a code that means we move $3$ places to the left to discover what the code stands for. By the way, when it comes to A B C being coded they reach back to the end of the alphabet so that the letters go,
SDW - D - FDNH,
WKH - ZHVW - EORZV,
BRX - DUH - ZRNLQJ - KDUG.