Is there any list of operators and functions which can be used in formulas?
I am trying to assign game points based on passed variable, but never go below 3:

max(3, $variable["storypoints"]/5)

Nop, currently we don't have these functions as a part of our variable system.

However, not sure if it works for your use case, but you can always assign a minimum value to a metric white creating it.