Thanks for directing me to Scanlon’s work. I am adequately familiar with his view on this topic, at least the one that he puts forward in What We Owe to Each Other. There, he tried to put forward an argument to explain why we should save the greater number in a choice situation like the one involving Bob, Amy and Susie, which respected the separateness of persons, but his argument has been well refuted by people like Michael Otsuka (2000, 2006).
Regarding your second point, what reason can you give for giving each person less than the maximum equal chance possible (e.g. 50%) aside from wanting to sidestep a conclusion that is worrying to you? Suppose I choose to give Bob, Amy and Susie each a 1% of being saved, instead of each a 50% of being saved, and I say to them, “Hey none of you have anything to complain about because I’m technically giving each of you an equal chance, even though most likely, none of you will be saved.” Each of them can reasonably protest that doing so does not treat them with the appropriate level of concern. Say then, I give each of them a 1⁄3 chance of being saved (as you propose we do) and again I say to them, “Hey none of you have anything to complain about because I’m technically giving each of you an equal chance”. Don’t you think they can reasonably protest in the same way until I give them each the maximum equal chance (i.e. 50%)?
Regarding your third point, I don’t see how I can divide up the groups differently. They come to me as given. For example, I can’t somehow switch Bob and Amy’s place such that the choice situation is one of either helping Amy or helping Bob and Susie. How would I do that?