The third type - what can no longer be changed
The first conditional is real (Si j'ai le temps, je viens.), the second plays with something in the present (Si j'avais le temps, je viendrais.). The third is about the past: what did or did not happen can no longer be changed. That is why it is used less for weighing options than for regret, blame and justifying yourself.
Si j'avais su, je ne serais pas venu.
If I had known, I wouldn't have come.
Si nous étions partis plus tôt, nous n'aurions pas raté le train.
If we had left earlier, we wouldn't have missed the train.
Si elle était restée, tout aurait été différent.
If she had stayed, everything would have been different.
What it is used for
Regret and justification
About your own actions: what you would have done differently, or why you acted as you did.
Si j'avais étudié le droit, ma vie aurait été autre.
If I had studied law, my life would have been different.
Si j'avais su, je t'aurais attendu.
If I had known, I would have waited for you.
Je ne serais pas venue si on m'avait prévenue.
I wouldn't have come if somebody had told me.
Blaming others
About what somebody else did: had you acted differently, things would have turned out differently.
Si tu avais fait attention, ça ne serait jamais arrivé.
If you had paid attention, it would never have happened.
Si tu m'avais prévenu, j'aurais pu t'aider.
If you had warned me, I could have helped you.
The si-clause alone
In conversation the main clause is often dropped. The si-clause alone is then an exclamation of regret, like English If only I had known!
Ah, si j'avais su !
If only I had known!
Si seulement tu m'avais écouté !
If only you had listened to me!
Type 2 or type 3?
The time frame fixes both halves. Type 2 plays with something in the present (imparfait + conditionnel présent), type 3 talks about something past (plus-que-parfait + conditionnel passé).
Si j'avais de l'argent, je voyagerais.
If I had money, I would travel.
Si j'avais eu de l'argent, j'aurais voyagé.
If I had had money, I would have travelled.
