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.