For 3 years of experience in Java, prepare the following concepts for Java Interview:
Core java
OOPS (specially differentiating abstraction & encapsulation)
String (immutability), String Buffer, String Builder
Threads (implementing ways, race around the condition, deadlock, etc.)
Exception Handling
Object class (hash code and equals methods)
Comparator, comparable
Interface, abstract class
Collections (differences and when to use)
Static class, static method, static block difference
Final keyword - significance and use
Heap area, stack area, string pool
Singleton class and its implementation
Advanced Java
Difference between JSP, Servlet
Forward and redirect
Life cycle of JSP and servlet
JSP scriptlets, directives, declaration, expressions
If you are preparing for an interview, I would recommend this Java Interview Questions blog for the most frequently asked questions in Java Interview.