'What's the difference' vs. 'What are (the, their, these, those) differences'

[Lee12jan]

Dears:

1) when there are over 1 difference between 2 objects, should we use 'What are differences between xx and yy' ?
2) when we compare over 2 objects, should we use 'What's the difference among xxx and yyy and zzz' ?
3) When will you use 'What are differences...' and can we add (the/their/these/those) ?

[Torsten]

What is the difference between xx and yy?
What are the differences between xx and yy?

What are differences between xx and yy? This is incorrect.
What are differences among xx and yy? This is incorrect.

TOEIC listening, talks: New work schedule

[Lee12jan]

Dear Torsten, thanks for your response. BTW, below is right ?

What is the difference among xx and yy and zz ?
What are the differences among xx and yy and zz ?