There are some basic rules when it comes to buttoning your suit jacket. Starting with a single button suit you must always button the only button when standing and unbutton when sitting. Next, when you are wearing a two button suit you must only button the top button while standing never both buttons. Lastly, if you ever find yourself wearing a three button suit jacket starting with the top button and working your way down it’s sometimes appropriate to have the top button buttoned along with the middle one. It is always appropriate to have the middle button buttoned (the middle button pulls the jacket together at your natural waist and lets the bottom naturally flare out around your hips), and you should never button the last button as doing so messes up the intended tailoring and flare offered by the middle button. They call this the sometimes, always, never rule. Plain and simple.