It depends. If you want a relax and casual look, you will only need to button the middle one since the structure will hide the top button and give you a deeper drop on the neck. If you want a more conservative and serious look, you will button the top two. You will see the big difference. It is a more serious, professional and severe look.
The 3-button suit I have doesn't really give a deeper "V" without buttoning the top button, as the structure of the suit pretty much dictates the way it hangs. Maybe this is different for other 3-button suits though.
To the OP, if the top button is rolled behind the lapel, definitely don't button it.