I hate these ultimatums. Real life doesn't work that way with choice A, B, or C being your only options. In such a hypothetical scenario I would probably cause the lifeboat to sink attempting to save all three of us. I refuse to accept the set of rules.
First, I don't know the future. I wouldn't just throw someone off to ensure I would survive, given a chance that anything could happen up until the boat actually sinks. Second, ethically either jumping overboard or throwing someone overboard is proactive and condemning someone to die. I'd do what I could to find another way, I don't play the game.