It must of been good is improper English. Always say ‘it must have been good’.
Example of correct usage of their/there/they’re: Their (indicates possession) dog is over there (location) where they’re (abbreviation of they are) playing ball.
Example of correct usage of your/you’re: You’re (abbreviation of you are) looking good in your (belonging to you) new dress.
You scratch an itch: The itch is the sensation, the scratch satisfies it.