Basics to Encrypting

[pikapika]

I understand what the general function should be but I don't know how to "move letters"

•Look at each group of 3 characters. Move the third character to the first

ABCEFG ----> CABGEF

•add an adjustment value (entered by the user) to each letter

adjustment: 3

ABCDEFG ---> DEFGHIJ

Does this mean I should set every letter in the alphabet to a number and set up and equation? Or is there some other way to do this?

A = 1
B = 2
C = 3
...
Y = 25
Z = 26

I'm really new to coding, thank you for your patience if this is a silly question.

Any help is appreciated! Thank you!