Use Third Conditional to talk about UNREAL situations in the PAST - things that DIDN'T happen. You're imagining how the past could have been different. Structure: IF + PAST PERFECT, WOULD HAVE + past participle. Use for regrets, criticism, and imagining different past outcomes.
Practice tip: Is it about the PAST that DIDN'T happen? Third Conditional! IF clause: had + past participle. Result: would have + past participle. You're imagining a different past!