write/approve post permissions

hi, I’d like to start a blog and I’d like to have three sets of permissions:

1) write posts AND approve other peoples’ posts (myself and a few others)

2) write posts but NOT approve other peoples’ posts (these posts would be approved by group 1) above)

3) not write posts, only comments

is this possible? how do I find out how to do this?

thanks!
*Stephanie