What is the difference between 差别, 分别, and 区别?
When they function as nouns to mean difference in the general sense, they are mostly interchangeable.

这两个东西的差别/分别/区别在于……
The difference between these two thingies lies in…

差别 seems to emphasize the difference itself, while 分别/区别 tend to indicate a desire to tell two different things apart (therefore the word &quot;difference&quot; in the title of your question should be translated into 区别). The above example sentence may have slightly different nuances depending on interpretation:

这两个东西的差别是……
What is different in one than in the other is…
这两个东西的区别是……
What distinguishes one from the other is…

BTW, although not in noun form, 差别对待/分别对待/区别对待 makes for an interesting comparison:

差别对待
to treat discriminatorily
分别对待
to treat separately
区别对待
to treat discriminatively (not necessarily with discrimination)