The term 'Quit Yer Bellyachin'' is a phrase used to express the idea of not complaining or whining about something. It is often used as a humorous way to tell someone to stop complaining and take action instead. The three letter solution for this crossword is 'YER', which is a shortened form of the word 'YOU're'. Therefore, the full phrase can be interpreted as 'Quit YOU're Bellyachin'', which is an informal way of telling someone to stop complaining and take action.