Mark didn’t apply for the job because he didn’t get his diploma on time.
i’m not sure if i should use conditional 2 or 3.
personally, i think it’s 3. because it is contrary to reality.
if mark had had got his diploma on time, he would have applied for the job
This situation has already happened in the past, so it is an impossible condition. Therefore, you’d use the third conditional, as stated in your post. The second conditional is used for imaginary situations in the future, which are still possible but unlikely: “What would you do if you won the jackpot?”. In this case, it is impossible for Mark to get his diploma on time since it is a situation in the past which is now over.