I'm not a native speaker either, but I agree with Kendall that the three French verbs mean go to bed, fall asleep, and rest respectively.
What I do feel I need to point out, while I'm here, is an obvious mistake in your English. Your question should be this:
What's the difference between these verbs in French?
English learners often make the mistake of thinking that the expression is What's the different. This is wrong, and is also an error which no native English speaker would make.
Now, according to your profile, you are a native English speaker. It's entirely up to you what you choose to call yourself. But please remember that there are hundreds of English learners here who'll see your profile and will presume that your English is correct. It's not fair on other italki members to mislead them in this way.
I am not a native French speaker but have a very high level.
My understanding is the following:
Se coucher: to go to bed
S'endormir: to fall asleep
Se reposer: to rest
I hope this helps, and native speakers feel free to correct me!
no need to judge and being this tough thou