On a standard dartboard, the number directly opposite 20 is 3.
The familiar dartboard arrangement places 20 at the top and 3 at the bottom. The sequence is designed so that high numbers are generally separated by lower numbers, increasing the penalty for inaccurate throws. The conventional order is credited to Brian Gamlin, although details about his life and the exact history of the numbering system are uncertain.
The board has 20 numbered sectors arranged around the outer ring. Opposite sectors are separated by ten positions, so 20 and 3 form a vertical pair when the board is mounted with 20 at the top.
This fact is different from asking which number is at the top, a common introductory darts question. Board orientation matters: if a board is rotated incorrectly, the apparent opposite number may not match regulation presentation.