How do BPG, carbon dioxide, and protons affect hemoglobin's oxygen binding curve?

They all shift the curve to the right, decreasing hemoglobin's oxygen affinity.

BPG has no effect, while carbon dioxide and protons shift the curve to the right.

BPG shifts the curve to the right, while carbon dioxide and protons shift it to the left.

They all shift the curve to the left, increasing hemoglobin's oxygen affinity.