Business Analyst :  A person who is responsible for gathering requirements when building a software. Basically you are the middle man between the developers and the stakeholders. You don't have to be technically sophisticated to be a business analyst but again depends on the company. The last company I worked as a  business analyst. Most of the time I was involved in gathering requirements and testing it. 

System Analyst : System analysts are similar to business analyst but are more technically sophisticated. They are the subject matter expert in the system they are working. 

Data Analyst : I have no experience as a data analyst but it would be safe to say  that they deal with data projection and evaluation :)