In Guns, Germs, and Steel, what are the three objections Diamond discussed in relation to Yali's question?
The answer to this can be found in the Prologue of the book. It is found on pages 17 and 18 in the paperback edition. What Diamond is talking about here is not his own objections to Yali’s question. Instead, he is giving reasons why other people “take offense at the mere posing of the question.”
Diamond gives, as you say, three objections that people sometimes raise. The first of these is that explaining why Europeans came to dominate would be the same as justifying their domination. People say explaining the dominance would imply that there is nothing that can be done about it. The second objection is that answering this question is a way of glorifying Europeans. People who raise this objection think that we pay too much attention to Europeans when their dominance is not a permanent thing. The final objection is that answering Yali’s question implies that civilization is good and that earlier forms of organizing society are bad. It implies that civilized people are in some way better than people in other forms of society.