What's the difference between PRETTY MUCH, SO MUCH, VERY MUCH,? Is all of that the same?
PRETTY MUCH is new to me, I didn't know that I can use it.

Question

Answer

No, they all do not mean the same thing. 

1.Pretty much is what you say to agree with someone. 
Pretty much is similar to saying "that's right" or "basically." 

2.So much and Very much mean the same thing. They both 
epress "how much" of something. 

For example: 
"I love you so much" --->this is correct 
"I love you very much"--->this is sometimes correct 
"I love you pretty much"---> this is incorrect and you would not say it. 

So basically, you use pretty much to agree with someone. In contrast, 
you use "so much" and "very much" to express "how" much of something,

Answer

so much = very much

pretty much means 'to some degree'

'They are pretty much in love.'

Answer

June's answer is excellent. I would like to add that "pretty much" is also an It is an intensifier, indicating that whatever is referred to. will probably be/is that way.

Example:  Italki is pretty much more useful than I first thought!