I thought you could solve this simply by thinking about it. Ethylamine or rather ethylammonium ion has a pKa of 10.5, so it is 50% protonated at pH 10.5, ~10% at 11.5 and ~1% at 12.5. This should tell us both amines are protonated in B at pH 3. It must either be a cation or a di-cation. Not B.
They are all alpha amino acids, so this will be in common. You can determine how the alpha-amino group affects the carboxylic acid. The other ionizable substituents are a phenol (pKa 10), a carboxylic acid (pKa ~4.85), and an imidazole (pKa of imidazolium ion 7). Which groups will be uncharged at pH 3.0?