To do, to be doing, and doing

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. It’s nice to meet you!
  2. It’s nice to be meeting you!
  3. It’s nice meeting you!

Are they all correct? What’s the difference?

Thanks

2

It’s nice to meet you - applicable in most situations.
It’s nice to be meeting you - sounds odd and stilted.
It’s nice meeting you - ‘It was nice meeting you’ is often used when parting after meeting someone, but not ‘it is…’. When spoken, the speed of speech, etc. often makes ‘it was’ sound identical to it’s (perhaps slightly more it’z) as we have discussed previously.

3

it’s simple!

4
  1. I am happy to sit here with you.
  2. I am happy to be sitting here with you.
  3. I am happy sitting here with you.

What about these three?

Thanks

5

All have very similar meanings. the differences between them are subtle.
Here’s my interpretation, but these subtleties tend to be personal. Other native English speakers may have slightly different interpretations:

happy to sit - emphasises the action of sitting.
happy to be sitting - emphasises the company ‘with you’.
happy sitting - I don’t mind doing this, I have no urge to do anything else.