In 3 Way bets, there are three possible outcomes and you have to pick the right one. The team you choose to win must win by the end of regulation. If the game goes into overtime or a shoot out you lose the wager, unless you have selected the tie.
3 Way bets usually refer to the result at the end of the regular playing time (exceptions are stated in the General Terms and Conditions or are announced as they occur).
For example in Arsenal vs. Manchester if you chose three way Arsenal and the game goes to overtime 2-2 and then Arsenal wins, you still lose this wager.