I think I would say: - We need to do something if we want to stop THIS from happening again. (minor tweak on your number 2) - We need to do something if we want to prevent this from happening again. (you can use prevent instead of stop in your number 2) - We need to do something if we don't want this to happen again. (your number 3) - We have to do something if we ... (any of the above can follow)

All three sentences sound natural, but the second one is the most straightforward and commonly used. Here's a breakdown: 1. "We need to do something if we want to prevent it from repeating." * Sounds correct but a bit formal. The word "repeating" might feel slightly less natural compared to "happening again." 2. "We need to do something if we want to stop it from happening again." * Most natural and commonly used in casual conversation. 3. "We need to do something if we don't want this to happen again." * Also natural, but slightly more casual and conversational than the second one. Recommendation: Use the second one for clarity and simplicity.

3rd sentence

The second one.