What's up? How is it going? What's going on?
When can they be used?

Hi Rod,

These questions seem very similar but they are used in different contexts:
What´s up? is used to ask someone what is wrong or what is going wrong with them, that´s to say, if they have any problem.
How is it going? is used as a friendly greeting. It´s the same as How are you?
What´s going on? is nearly the same as What´s the matter? and it is used in the moment of speaking, that´s to say, what is exactly happening.

Thank you nuhru_1098 fot the explanation.

Hi ,

you´re welcome.
I hope I helped you with my explanations. Sorry for my delay but I´ve been correcting lots of examinations these days.
See you.