What will happen if I visited a virus-infected site for about 3 secs?

Theoretically, as a techy I can confirm that it only takes a split second for something to happen, but this is testament to the importance of antivirus and firewall software. If you don’t have these, then every OS comes with utility tools to maintain systems health, you can consult them if you feel the need but you’d be a lot better if you dealt with it head on by running a thorough scan over night or something. Like I said, it only takes a second, but it’s just as likely that you might have dodged it in time with just 3secs.