Faceit uses http api 'https://api.faceit.com/core/v1/nicknames?nickname=' + username to find if a username is valid. Code is just some python hack to bruteforce 3 letter usernames
PS C:\Users\X\Documents\Tools> python .\UsernameFinder.py
[~] Found: agw
[~] Found: ahq
[~] Found: ahv
[~] Found: aqg
[~] Found: aqj
[~] Found: aqn
[~] Found: ayp
[~] Found: bcu
[~] Found: bdu
[~] Found: bfq
[~] Found: bfu
[~] Found: bgi
[~] Found: bhh
[~] Found: bhv
[~] Found: bjy
[~] Found: bjz
[~] Found: bmi
[~] Found: bpi
[~] Found: bpu
[~] Found: bqc
[~] Found: bqf
[~] Found: bqg
[~] Found: bqh
[~] Found: bqo
[~] Found: bqp
[~] Found: bqs
[~] Found: bqx
[~] Found: bqy
[~] Found: buv
[~] Found: bvf
[~] Found: bvj
[~] Found: bvq
[~] Found: bvu
[~] Found: bvw
[~] Found: bwo
[~] Found: bwq
[~] Found: bwv
[~] Found: bwx
[~] Found: bxf
[~] Found: bxo
[~] Found: bxw
[~] Found: cdq