Hey Ringo! Welcome to TB!
To answer your main question: yes you can play either the C/E/G and it will sound "right" (Right meaning it will sound pleasing to most ears, but there is no actually right/wrong in regards to what your asking), but they'll all add a different quality to the sound. Its really dependent on what you are looking for in the song..
I'd agree with the above, try it and see how it sounds first. If you don't like it, try the next note and so on.
If you want more info to help you out go to
http://www.studybass.com, I found this site while reading posts here on TB and it has a lot of great content! (Not affiliated in any way)
Hope that helps.