I don’t know exactly when they came but Seungmin joined in 2017 so Felix and Minho’s training period must only be a few months. However, if I remember correctly, Felix waited to finish school before flying to Korea so he was accepted into the company for a while before actually training with skz. And Minho auditioned a few years back (2016?) after being scouted at some dance event but was only contacted again in 2017. JYPE may also have been aware of his career as a backup dancer as JYPE and Bighit sometimes work closely (a recent example is Itzy members acting in BTS videos when they were trainees). So while their training periods were short they had interactions with the company for a longer period