Recommendation letter from advisor with different research interest
I think it depends on exactly how long you worked with these two individuals and what you accomplished. You want strong letters, so if you worked with both of them long enough for them to assess your basic research abilities and potential to develop into an independent researcher, I think the fact that they don't work in your exact desired field won't matter much. The fact that all three topics fall under the same Economics umbrella probably makes it even less of an issue.

It is true that sustained interest in a field/topic is usually a positive. However, I think you and your recommendation letters can spin even short-lived interests into a positive thing if you demonstrate that your prior experiences in A and B were valuable from a research skills development standpoint, while also helping you narrow your research interests down to topic C. Switching topics after you start a PhD can become difficult (especially the longer you wait), so admissions committees may look favorably on any prior work that helps you discover your research passion.