If a man says to his wife: "By God, by God, by God, if you leave the house before we return from the trip, you are divorced with three pronouncements," is this considered a conditional divorce or an oath that can be expiated? And if it is a conditional divorce, does one pronouncement of divorce occur, or three, if she leaves before returning from the trip?

What you uttered is not a conditional divorce but an oath. If your wife does not leave before your return from your journey, then there is nothing upon you. However, if she does leave, you have the choice between expiating your oath (which is preferable if it is in the interest not to divorce, as the Prophet, peace be upon him, said: "If one of you takes an oath and then sees something better than it, let him expiate it and do what is better"), or divorcing her thrice to fulfill your oath. If you neither expiate nor divorce, nothing is binding upon you unless the divorce becomes impossible due to the death of one of you, in which case expiation becomes obligatory.