Tips on multiple key Map-wrapper
Some notes about the current code:


extends Objects looks unnecessary in the class declaration:

public class MultiMap&lt;K1 extends Object, K2 extends Object, V extends Object&gt; {


I'm not completely sure but the following seems to be the same:

public class MultiMap&lt;K1, K2, V&gt; {

I'd rename res to oldValue or something more descriptive.
Modern IDEs use highlighting to separate local variables from fields, so don't need to use this.. It's rather noise.
Comments like this are just noise:

 /**
 * @param map1
 * @param map2
 */


It doesn't say anything more than the code says. Furthermore, the constructor does not have any parameter like map1 or map2. (Clean Code by Robert C. Martin: Chapter 4: Comments, Noise Comments)