This would be a
good perfect use case for Multi Site Manager.
It sounds like you haven't gone that route though and retrofitting MSM can be a bit of a pain as far as I remember (only done it once).
Using MSM you would need to:
1. Set up the channel in exampleone
2. In your channel:entries tags on the other sites add the site's ID to the channel parameter.
If you wish to mix these entries in with some from the other sites you may want to also read about orderby and multi site manager.
Alternatively you could perhaps publish an RSS feed of the stories from site exampleone and pull them into the other two sites.
Create an RSS template on exampleone that uses a standard channel entries tag pair to write out the data in an RSS format.
Install something like Feed Parser on the other sites and give it the URL of your RSS feed from exampleone.