Not only is 3 one of my favorite numbers it is also a good number to know when decorating your home. 3 is the number that creates pleasing proportions in your home.
1) Accessories should always be placed in 3's. 2's just never look right.
2) Chair rails should be placed a 1/3 of the way up a wall.
3) Tie your draperies a third of the way up.
These probably are already instinctual to you but now that you know the rule it'll hopefully make you feel more confident that you are making the right decisions.