so i have a server running pocketmine with the api 3.0.0. most plugins i saw where usefull but with older api, how to change the api version from 2.1.0 to 3.0.0? or can someone tell me where to find plugins with the 3.0.0 api
You can't. API 3.0.0 has major changes. You won't be able to use plugins from older APIs with it. We (PMMP Team) also don't recommend releasing any plugins using API 3.0.0 at the moment, because the API is not finalized, and the API may change in the future, very likely to make your plugins no longer work.
In other words, if you are a developer and want to mess about with development API, have fun, but expect your plugins to break as soon as a new development build rolls around.