in example 1 you are checking to see if you have a mouse move
event on the _root timeline.
in example 2 you are checking to see if you have a mouse move
event on the object you have the script on.
In example 3 you are doing the same thing as example 2
They all work the same for you because you probably have both
1 and 2 on the main timeline so _root and this are the same thing.
If you had example 1 on the main timeline and example 2 on a movie
clip then they would not be the same because the 1st would check
for mouseMove on the stage and the second would check for mouseMove
of the image.