What happens in h20 just add water season 3?

Answer 1

According to Wikipedia, Bella joins the group replacing Emma. Bella is also a mermaid. The three join to find a source of a tentacle attacking them.

Answer 2

In season 3 of H2O: Just Add Water, the main characters Cleo, Rikki, and Bella face new challenges as mermaids, including dealing with a powerful water tentacle and discovering the existence of a mermaid secret society. They also navigate the ups and downs of their personal lives and relationships while continuing to keep their identities as mermaids a secret.