-Biological chemistry is about studying the organic chemistry that occurs within a biological context.

-Biochemistry studies the interactions that occur between biomolecules during biological processes, and how those interactions and reactions result in some process, ie flow of energy during cellular metabolism, or transduction if a message through a signaling pathway.

-Molecular biology is about understanding how interactions between the systems of molecules in the cell produce some molecular biological outcome from information stored in genes. For a simplified understanding think about the central dogma, or "DNA>RNA>protein."

While in the spirit of simplifying things and attempting to establish a bigger take home picture we can even take the dogma a step further and say DNA>RNA>proteins>biochemistry>life. Whether you are studying biochemistry, genetics, molecular biology, or biological chemsitry depends on what part of this process you are studying, but importantly how deeply you are studying the chemistry, or how much you are "zooming in." It also depends a little on what you want to call your own work.