You can get the average of your tracks at Output tab in the tracker or by simply entering the sum of all of trackers and then dividing the result by the total number of tracker in the sum
=(.tracker#.+.tracker#.)/
Like
=(Tracker1.tracker1.x+Tracker1.tracker2.x+Tracker1.tracker3.x)/3
This will give you the average of trackers 1, 2 and 3
The simpliest way to stabilize in Nuke is by clicking Generate Stabilize Node on the Tracker main tab, but you can also use the Stabilize2D from the transform menu or by turning your tracking data into negative values at any input field.
Corner pinning all you need is to copy or link your tracks to the four corner x and y values.
I believe you can find all of these infos on the manuals 