Coins are placed within a rectangular grid. Some are showing heads, others tails, at random.

You can select coins, three at a time (three adjacent coins in a line, horizontally, vertically or diagonally), and turn them over. Through a sequence of such moves, it is possible to make it so that all of the coins are showing heads.

Find the arrangement with the fewest coins, such that any starting arrangement of heads or tails can be made all heads by a series of three-in-a-row flips in horizontal, vertical or diagonal directions.

The coins must lie in a rectangular grid pattern. You can have spaces in the grid without coins, however a triplet of coins you wish to flip cannot bridge across any gaps.