Eva,
It's not clear what all of your actual business requirements are. So, I will just make some technical comments. You can change the activities performed by a planning procedure. SCM IMG > APO > Supply Chain Planning > PPDS > Maintain Planning Procedure. Create a new Planning Procedure that meets your business requirements, and then use the custom PP in your product masters instead of PP 3.
Standard PP 3 during "Create or change a sales order" runs the product heuristic. Standard PP 3 during "Change sales order confirmation" also sets the planning file entry. You can experiment with changing or removing some of these activities in your new PP. If you elect to continue automatically executing the product heuristic, you can also create a custom heuristic that will perform exactly the Planning activities you wish for it to perform.
Best Regards,
DB49