pirate

None of these 3 letter-only usernames below are available anymore as of 2021.
However, if you use numbers and symbols, or accept 4 letters, you can definitely find a free one.
Check responsibly. Don't spam the github API/support, it's not a race, there are plenty free if you have imagination.

agq, ahq, aqf, aqg, aqp, aqt, aqf, aqy, atq, auh, ayp, azj, azq, bey, bgt, bgx, bhq, bkk, bkq, bmq, 
bpp, bpq, bqa, bqc, bqg, bqi, bqj, bql, bqn, bqo, bqp, bqr, bqt, bqy, buo, buq, bwz, bxe, bxo, bxw, 
bzn, bzp, cfl, ckg, ckq, cnq, cpq, cpz, cqa, cqe, cqf, cqg, cqk, cqo, cqp, cqf, cqx, cqz, cud, cuh, 
cuk, cuo, cfl, cxe, czo, dkq, dnq, dqg, dqi, dqk, dqo, dqs, dsr, dtq, dxe, eaj, eaq, ebq, ecl, ecy, 
eer, efq, efy, egq, egx, ehh, ehz, eiu, eiw, eiy, ejx, eoq, eou, epj, eqa, eqb, eqf, eqg, eqj, eqk, 
eqs, eqf, eqw, eqz, erq, etq, eub, euf, euj, euq, euf, efq, efy, ewy, ewz, exn, eyh, eyj, eyn, eyq,