Recommendation letter from advisor with different research interest While I was an undergrad, I wrote a paper on topic A with my advisor. Currently, I'm working in a research institution with an Economist whose work mainly lies on topic B. However, I've come to understand that I do not like A or B, but I'm more interested in topic C, and I've done some independent study on topic C. I'm thinking of asking my undergrad advisor and my current Economist for recommendation letter to apply for PhD in topic C. 

How will admission committee for PhD view such recommendation letters from academics whose work is not on my current interest? That is, will it have any negative influence in my application, if I seemed to change my interest often? 

To be specific, all topic A, B and C is under the field of Economics, but are vastly different from one another (especially the switch from A to B; it was like learning completely new subject). Knowledge gained from A and B can only be partially transferred to C. 

Related, but does not quite answer the question that I have: Value of recommendation letters from professors in a different field. My question is more focused on the fact that two recommendation letters will show my interest in certain research topics were very short-lived.
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.