Are you
Circumference?
I am a knight, though in name not deed.
Sir Cumference! Haha
One of my friends can cut a swath through the land,
Diameter
And the other is a master at getting straight to the core.
Radius
We're fans of desserts, preferably apple or cherry.
Pi
But alas, I've no time, my work is never done.
Because you’re a never ending circle
So, good traveller, what is my name?
Hopefully circumference!