Sure, let's discuss the different pieces of advice given and decide which three are the most important for staying safe online.

1. **You shouldn't post too many photos.**
- Posting too many photos can reveal personal information and make you vulnerable to identity theft or stalking.

2. **You ought to use different passwords.**
- Using different passwords for different accounts is crucial because if one password is compromised, it won't affect all your accounts.

3. **I would advise you not to chat with people you don't know.**
- Chatting with strangers online can be risky as they might have malicious intentions or try to scam you.

4. **I would advise against using public computers.**
- Public computers can be insecure and might have malware or keyloggers that can steal your personal information.

5. **You'd better not reply to offensive comments online.**
- Engaging with offensive comments can escalate conflicts and expose you to further harassment.

6. **You shouldn't download attachments or files from people you don't know.**
- Downloading unknown attachments can lead to malware infections or phishing attacks.

### Most Important Three Pieces of Information:

1. **You ought to use different passwords.**
- This is crucial because it protects your various accounts from being compromised if one password is leaked.

2. **You shouldn't download attachments or files from people you don't know.**
- This is important to prevent malware infections and phishing attacks, which can lead to significant data breaches and financial loss.

3. **I would advise you not to chat with people you don't know.**
- This helps in avoiding scams, phishing attempts, and potential harassment from strangers with malicious intentions.

These three pieces of advice are fundamental in maintaining online safety and protecting personal information from various threats.