The most recent paper (and the only one from my hand about Cadabra 2.x) is
@article{Peeters2018,
doi = {10.21105/joss.01118},
url = {https://doi.org/10.21105/joss.01118},
year = {2018},
publisher = {The Open Journal},
volume = {3},
number = {32},
pages = {1118},
author = {Kasper Peeters},
title = {Cadabra2: computer algebra for field theory revisited},
journal = {Journal of Open Source Software}
}
So that's probably the best one. Wouldn't hurt me if you would cite the two others as well though.