Past Unreal Conditionals

Grammatically, the Past Unreal Conditionals have two important elements: the IF Clause and the MAIN Clause. 

IF Clause + Main Clause = Past Unreal Conditional = Third Conditional.

The IF Clause is formed by: IF and a sentence in Past Perfect.

IF Clause + Past Perfect = IF Clause

The Main Clause  is formed by: Would have / Could have / Might have and a Past Participle.

Would have + Past Participle = Main Clause.
Could have + Past Participle = Main Clause.
Might have + Past Participle = Main Clause.