Listing authorship on a resume
If your resume has a text statement about this work, describe your contribution, not your authorship position.
The point of author ordering is to give a rough guide of level of contribution.  But if you are actually describing the work in words, you can do better than that: give specifics that highlight the importance of your role.  Examples:

Was the primary person responsible for developing algorithm X.
Designed and executed experiments showing Y.
Conceived of the original idea for the work and contributed to experimental design and statistical analysis.

This is much better information about what you did.
If in some context you need a way to describe your authorship position in words, you could call yourself:

Co-first author (along with A and C).

You would need to mention A and C so that it is clear you are one of three, not one of two.
However, I'm not convinced that people will give you much credit for being one of three &quot;first&quot; authors.  I probably wouldn't bother to mention it.