I'm going to end a 3-year friendship with no explanation
Is there a good reason why you would end that 3 year friendship?
There is no reason behind that, I just feel our friendship has been going on for too long
There's no reason why friendships should end just because time has passed.  If you are no longer having as much fun together, you can remain friends but hang out with others.  It's wrong to just suddenly end things without telling your friend, and it will hurt them when they think that they did something wrong.
Why does it have to be explained?
Every hurtful decision needs an explanation. It's closure for both parties, so no bad feelings in the future.