We are building extranet, and we need to have 3 kind of user groups and don't know, what to do,

please give a hint or solution or address or someting...

I need three different user groups -> all of them are registered users. Group 1 is allowed to read only level 1 articles. Group 2 is allowed to read level 1 and 2 articles. Group 3 is allowed to read the levels of 1, 2 and 3 articles. How can I do this with K2? Is it possible to do with K2?