Equal author contribution
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.