I'd pass on all three.
Bella is an obvious nn but doesn't work for me as a full name.
Isabella has a rhythm I dislike, and
Isabelle - where I live anyway - is pronounced exactly like
Isabel, which I prefer: it's neater and tidier. I'd consider using
Isabel, but I'd be much more likely to use
Isobel. And, yes, no nn would be necessary; though I knew two girls at uni who were very good friends, always together, known as
Bella and Iz.