Hi Gabriele,
As per your description, it seems you want Conditional Formatting with Icon Sets using relative reference. Create a conditional formatting in a column that will look to the value in the previous month column e.g. if value higher than reference value then green, if equals then orange, otherwise red. You can use OFFSET formula e.g. in the conditional formatting, using the formula "=OFFSET($C$3,ROW()-3,0,1,1)" for the "green".
Here is for your reference: OFFSET function.
If any other community members have any better suggestion, please share your good suggestion here with us we will highly appreciate your shared suggestion.
I appreciate your time and understanding.
Best Regards
Waqas Muhammad