The rules of tournament poker etiquette say that in a 3 way pot with one player all-in, the other two players check their hands down, unless one player significantly improves his/her hand. The worst outcome would be for one of the hot-all-in players to bet and the other not-all-in player fold what turns out to be the winning hand that could have knocked the all-in player out of the tournament.
In a cash game, this rule does not apply nearly as strongly. If you improve your hand, bet just as you would if one of the players was not all-in. If the other player calls, the money goes in a side pot.