Custom post type in loop after every x posts

Hello!

I’d like to loop normal posts and custom post type “feature” in one loop.

The thing is that I want the first post in the loop to be “feature” or normal post depending which was published first. And after that comes 2 normal posts and “feature” post as every third post like this:

FEATURE or POST
POST
POST
FEATURE (if the first item in the loop is POST this one is the newest FEATURE)
POST
POST
FEATURE
POST
POST
….and so on.

I searched Google like crazy but didn’t find any help there.