Equal author contribution I, along with two fellow doctoral students, am about to publish a paper supervised by our professor. Our professor is listed as the last author, which of course is fair as he supervised the project.

One of us, student A, is listed as the first author, which there is not disagreement on. Student B is listed afterwards, and I am the third listed author.

The ordering of student B and I is alphabetical, nothing more. This is also reasonable, as we all believe that our contribution has been the same.

However, it is not listed in the paper that our contributions are the same. Should I insist on this before submitting the paper?
In fields where authorship order matters, the difference between second author and third author is very little, pragmatically, in terms of how people will think about your contributions.  Neither is the first author (whose name will be noticed in every citation), and neither is the last author (in those fields where last = senior).

What you can do, however, is what many journals now require, where you add a section at the end of the paper with a name like "Author Contributions."  In this section, you write down exactly what each author did, e.g., 


  R.L. performed the experiments, T.X. ran spectroscopic analysis, B.G. supplied a critical DNA construct, R.L. and T.X. analyzed the data, T.X. and B.M. wrote the manuscript, and R.L,.T.X., and B.M. edited the manuscript.


This won't change the "first impressions" that anybody has of the paper, but anybody who wants to know whether you're a significant contributor or not will be readily able to find out.