The short answer is that the second conditional is for the UNREAL present/future and the third conditional is for the UNREAL past. More information here: https://www.ef.com/ca/english-resources/english-grammar/conditional/

Second conditional is when we talk about something that will probably not happen in the present or in the future. We are speculating about something improbable. "If I won ten million dollars, I would buy everyone in town a car." If + past tense + would + bare infinitive Third conditional is when we talk about the past and a situation that isn't true. "If I had fallen down the stairs, I would have broken my neck" If + past perfect + would have + past participle. The third conditional is a higher possibility if circumstances had allowed.