Can I secure a 3-digit rank in JEE Advanced if I do start today from scratch with a decent level knowledge of class 11th topics ??

Hello there,
JEE Advanced is a tough exam, and getting a rank less than 1000 is little difficult. I'm not scaring you, if you work hard and use your time effectively, then definitely you can get your desired rank. You need to devote enough time to your studies. Solve the previous year question papers atleast twice. It's very helpful since you get to know the trending questions. Give regular mock tests. But before JEE Advanced, you need to qualify your JEE Main. You can use our knockout package to get good rank in JEE Main. Here's the link: https://learn.careers360.com/knockout-jee-main-april/
Hope this answer helps you.
Good luck!!