Stock Count Sheet

I have made an excel workbook to monitor the amount of stock held in our parts room. Sheet 2 includes part numbers, part descriptions and quantities in store room. Sheet 3 records the person who takes an item including the quantity he is taking and the part number. Every time i take an item out on sheet 3, i want that to deduct from the same items quantity on Sheet 2. There will be on sheet 3 serveral lines that may repeat the same part over and over.

I need a VB code that will do this job for me, unless there is a simple way of doing it with Excel funtions.

Any help would be appreciated