The three mermaids are Bella, Rikki, and Cleo. This can be inferred from the given options, where the names Bella, Rikki, and Cleo are listed together as one option. The other options, Amy, Danny, Brianna, and Susie, Aqua, Summer, do not match the given names of the three mermaids. Therefore, the correct answer is Bella, Rikki, and Cleo.