I have 3 kindle devices linked to my account for my kids to read books as well. We each buy our own books on the one amazon Kindle account however at times we dont want to share our purchases onto each others device. Can this be done?
I am assuming that you are talking about Kindle Fire which runs (heavily modified) Android OS.
All purchases made on a Kindle are linked to whatever Amazon account you are signed in with, not the actual device. If you wish for your purchases to be separated, you will need to have a different Amazon account for your second Kindle Fire.
Now the problem is that there is no way to share purchases between accounts. As far as I know, you cannot add a second Amazon account to the Kindle (like you can with multiple Google accounts on plain Android.)