Your cover letter is the first thing employers see when they have a look at your papers. It is supposed to introduce you to them. It is different from your CV because informs your employer about your work experience rather than your academic achievement.
A letter of interest" is a fancy term for "cover letter."
A letter of motivation is supposed to focus on one’s desire or “motives” in the application. It therefore includes your interests and relevant activities and does not, like a CV, list your educational degrees in detail.
I hope that helps