In chess, Knights moves to squares a distance $\sqrt5$ away. Bishops move distances $\sqrt2$, $\sqrt8$, $\sqrt{18}$, etc. Both pieces are restricted to non-integer distance moves.
Enter the Knishop, a most potent chess piece capable of moving to any squares at non-integer distance: $\sqrt2$, $\sqrt5$, $\sqrt8$, $\sqrt{10}$, $\sqrt{13}$, $\sqrt{18}$, ...
Given a chess board of unlimited extent, place three Knishops such that they can't capture each other, while any piece being added to the board can be captured in a single move.