In Live Chess, the 'Draw' button is generally used to offer your opponent a draw, which may be accepted or declined. However, in the instance of Three-fold Repetition, you can use the 'Draw' button to claim a draw, without your opponent having an option to decline.
Once the position has repeated three times, make your move then click on the 'Draw' button. This ends the game as a draw.
In online or turn-based chess, once the position is repeated three times, make your move. Now, on the screen for that game, just below the notations box there will be text for you to click on that says 'claim game'. Click on that to claim the draw. This method also works for the 50-move rule, which states that the game may be drawn if 50 moves (2 ply) are played with no captures and no pawn moves.
Finally, note that in cases where the opponent has insufficient material to mate (lone King, King + Knight, King + Bishop, King + 2 Knights) a draw will be automatically declared where there is a time-out.
keywords: draw 50 move threefold three-fold 3-fold 3 repeated position claim draw drawn repetition fifty