Buttoning the suit with 3 buttons one has the option of buttoning
only the middle button in the manner of the old three-button cuts, or following the convention of buttoning every button but the bottom-most one Double-breasted suits are always kept buttoned. When there is more than one to-button (as in a traditional six-on-two arrangement), only the top one is to be fastened. Often, this is the only one that can be properly fastened, because the bottom to-button often lacks a corresponding interior flap button. In the case of arrangement such as the six-on-one, particularly popular when the double-breasted suit first staged a comeback in the 1980s, one has no choice but to fasten the bottom button as this is the only option.
Single-breasted suits are buttoned while standing. In a three-button suit, all but the bottom button can be done up, but if one it is the middle one, if two it is the middle and top ones. In two button suits it is appropriate to button the top button, but never the bottom button only. When sitting the jacket should be unbuttoned to prevent bunching. Three-button suits present a couple of special situations. In the case of three-button suits with lapels that roll over the top button, the top button should not be fastened. These are typically older three-button jackets; contemporary three-button suits are not normally designed this way. With a typical three-button suit made today, one has the option of buttoning only the middle button in the manner of the old three-button cuts, or following the convention of buttoning every button but the bottom-most one.
http://en.wikipedia.org/wiki/Suit_%28clothing%29#Buttoning_the_suit
