The first two sentences sound OK. The third one seems a little awkward to me. Sticking to the past unreal conditional tense, I would probably write: "Imagine the effect that could have had in their time or even in ours."

Hi, The first 2 sentences are fine; the third is missing a word. I agree with both postings before mine, as far as possible completions --it really depends on exactly what or who is making "magnitude". Perhaps these options would complete it for your needs: "Imagine the magnitude that that could have had in their time..." or "Imagine the magnitude that they could have had in their time..."

The first two sentences look grammatically correct. However, for the second sentence you would need to use a question mark instead of an exclamation point. Also, for the last sentence I think you are missing the word "happened". (Imagine the magnitude that could have HAPPENED in their time, as well as in ours.)