I need to know what are the major topics a 3 years experienced DBA must know in oracle to go for an interview
None at all to go for an interview.
To pass an interview? That's another matter but also depends on what the DBA is expected to do at that organization. If they are looking for a senior DBA you need to know everything: installation, configuration, backup and recovery, performance tuning, etc.
As a junior DBA they may just be looking for someone to deal with log and archive files and help with server-related issues (e.g. patches).
It all depends on what role the person is going to play, the level of that role, how many other DBAs are on the team and what the skill level of those other DBAs is.
Site dependent, but I would expect at least a cursory working knowledge touching on every chapter in the concepts and performance manuals, plus in depth knowledge of whatever you've done for the 3 years. I would expect some knowledge of what topics have been hot online, where the major online information centers are and how one would evaluate how good they are. Examples of how one works through bugs and interfaces (or avoids) MOS. Examples of how one deals with cow-orkers, especially difficult orks and how one positively deals with inevitable negative situations. Examples of how one deals with rapidly changing job tasks and how far across the technology stack you've spread as a DBA.
I have a dim view of interviewing though. More often interviewers either depend on some book of interview questions, or whatever whoever they are replacing worked on the last three years.
I need to know what are the major topics a 3 years experienced DBA must know in oracle to go for an interview.
I just need some pointers on this..
Such a question supposes a fallacy. Several fallacies, actually.
Life in general, and the world of work in particular, is simply not that black-and-white. And an interview is not simply a school exam cast in a different format. Job titles don't have fixed meanings. Your 3 years of "experience" does not equate to anyone else's 3 years of "experience". Employer A's expectation of someone with 3 years of "experience" (doing what, exactly?) may be far different that Employer B's expectation of someone with 3 years of "experience". You and I may have worked in adjacent cubicles for 3 years and I have 3 years of "experience" while you have 1 year's experience, 3 times.
You go to an interview with whatever knowledge and experience you have. If you "pass" the interview and get the job based on your ability to memorize a list of questions and answers (leading the hiring manager to think you know/understand more than you do) what are you going to do when (not "if") the excrement hits the ventilator and everyone is looking to you to save the company's crown jewels? We see it here all the time. You'll post a question to this forum, begging some stranger to tell you, in detail, exactly what you need to do. And oh, by the way, "it's urgent".