What is the difference between the three? I can see that in No 3 you are creating the new element as an anonymous object rather than giving it a name. I am not sure I can see a difference between 1 and 2. But you are not adding anything to an array; you have an object which you are adding the mappings to. Is that a List?
I suggest you compile all those three methods and print their bytecode with
javap -c. Please show us all three outputs. Can you actually see a difference?
You should not worry about such tiny bits of optimisation but I suspect you will find no difference in speed between the three.