Magnification is by far the best known of these three functions, but it really is the least important. Most things in space are so far away that no matter how much you magnify them, they still look pretty much the same, the only difference is they get dimmer. What really allows you to see more distant objects better is to make them brighter, this is done by light gathering . The larger the lens or mirror in diameter, the more light it's able to capture and the brighter the object appears. This enables you to see much more than you can by just magnifying.
Any good telescope can change it's magnification by simply changing the eyepiece.
The easiest way to understand light gathering is to think about the pupil in your eye. On a sunny day it gets small. In the dark it gets big, that lets more light into your eye so that you can see better in faint light, but your eyes can only get so big. This will allow you to see very faint objects. Resolution is perhaps the most difficult to understand.