Procedural style Rock Paper Scissors game using Python 3
I recommend simplifying:

if decision == 'rock' or decision == 'paper' or decision == 'scissors':


to:

if decision.lower() in ('rock', 'paper', 'scissors'):


This will check whether the decision exists in a given tuple.

This helps simplifying the idea "If a variable contains one of these values".

Another example is "If variable x contains either 1, 3, or 5." Instead of coding it like this:

if x == 5 or x == 3 or x == 1


It can be simplified into:

if x in (1, 3, 5)


See this Gist to see my suggestion as a whole.