How to use CheckBox for set string in Android

In my application i want use 3 CheckBox and i want when check of each this CheckBoxs add one world into my String.
For example : I want 3 CheckBox (chBoxFree, chBoxDownload, chBoxBuy) and when check each of this CheckBoxs add Free, Download, Buy into one String and when uncheck CheckBox remove from this String.
If checked the chBoxFree, chBoxBuy add "free-buy" in this string value and when uncheck chBoxBuy, remove buy from this string

How can i it?