Answer:
1. If i have to take part in a simulation, i wouldn't do it well.
2. If you placed the job advert online, you would get more applicants.
3. If we had used an agency, we could have saved a lot of time.
Explanation:
In finding the correct verb forms, we remember there is something called CONCORD in English.
Concord strives to find an agreement between a verb and its subject. If the verb is in present tense, the subject must be in present tense, same thing if it is in past tense.
From the 2nd sentence, the verb 'placed' is in the past tense, so it makes sense that the subject would also be in past tense. That's why the corrwctcorrect answer there is "would" and not "will"