Howdy
"Howdy" is an informal greeting.
It is a popular way to greet someone in southern and western states, especially in Texas.
What's up?
"What's up?" is American slang. It is a very informal way to greet someone.
What's up? is a popular greeting among young people and people who know each other very well.
How's it going?
"How's it going?" is an informal greeting. It is a casual and friendly way to greet someone.
People who ask this question are usually trying to start a conversation.