Splay tree amortized cost analysis

I am looking into the amortized analysis of splay trees and seem to be missing something.

Pretty much every resource uses the accounting method which I believe I grasp. What confuses me is the part where an upper bound is put on credits:

$$δ≤3(r′(x)−r(x))$$ where $$r'(x)$$ is the rank of a node after splaying and $$r(x)$$ is the rank before.

Could someone help me with there the $$3$$ is coming from? It appears to be common knowledge which I'm missing as everyone just uses it as given. Thanks!

• Can you edit the question to add a reference to that part that is easily accessible and likely to be stably available? – John L. Jun 10 '19 at 21:10