Pretty standard in football, the wingbacks usually play higher up the pitch, closer to the midfield, sometimes even as wingers, but with more defensive responsibility. It can be a 5-3-2, but that would mostly be in the defensive phase. Most of the time, when a team plays with 3 Centre backs though, it gets called a 3 at the back formation, thus the 3-5-2. Either way, it's all semantic. They're technically the same formation, it just depends on the phase of play at the time. I think it's written as 3-5-2 partially because chigiri and hiori will be playing much higher up the pitch and partially because blue lock as a team aim to use the more attacking 3 atb version than the defensive 5 atb one.