Saving checkbox_list as single row

I would like to be able to save the selected values from a checkbox_list as a single row in the database, so the output would look something like one,two,three.

The site I am working on uses a front end system for creating posts from logged in users, but the values are saved as a single row in the database which causes the values to be unchecked if they are viewed and saved again in the admin.

I have read through the documentation but have not been able to find a way of doing this.