[Grafana v5.2.3] Alert condiftion - diff() vs percent_diff()


I would like to understand how to use the diff and percent_diff() in the ALERT condition?
Can I check the value of last 1hour against last 3 days the same hour?

can I make use of these 2 functions? and configure the right parameter for the query(A, 72h,now)?

Kindly advise