Hi @tommasobriguglio. I am not sure we have enough details to be able to create a custom working solution for you.
If your part document contains a parameter named "Thickness", and that parameter is controlling the thickness of your part, does it contain an equation, or is it just a simple numerical expression? If it currently contains an equation, then setting its value to a simple numerical value within the iLogic rule may break its functionality. If it contains an equation, you may have to either modify its equation, or modify another parameter's value that is being used as part of the equation.
However, if that parameter exists, and is in control of the part's thickness, and just contains a simple numerical value, then something simple very similar to what you posted above should work.
If Thickness < 3mm Then
Thickness = 3mm
End If
If this does not work, then could you maybe post the part document here, so that we could get a better understanding of the situation. If you do post a part, make sure you remove any personal or proprietary information from it first.
Wesley Crihfield

(Not an Autodesk Employee)