Merge and add rows/values and stock adjustment

Hi, im looking for some help with a spreadsheet to keep track of stock.
Im only a beginner at excel really, so cant do much with VBA and cant quite get formula to do what i want.
So this is what im trying to do:

Sheet 1 will record the stock for everything, including descriptions etc.
Sheet 2 will be to fill in orders that are made.
Sheet 3 will be to show what stock needs to be ordered.

Everything needs to be linked and auto-update. i.e. if quantities are changed/orders come in etc.

Firstly i need to link the order sheet to the main sheet, so that when an order is received the stock updates. So the stock column (sheet 1) needs to search for any matching part numbers (sheet 2), and if they are received then add the ordered stock.

Secondly i need sheet 3 linked to sheet 1, so that all the parts are copied over with their descriptions etc. but if there are multiple parts appearing on sheet 3 then they will need to be merged and the quantites added together.
It would also be helpful the list is sorted so that any parts with qty to order of 0 are sent to the bottom of the list, and if possible a secondary sort by part number so those two sub sets are sorted by part number.

Any help would be greatly appreciated, macros or formulae

Can anyone help with this?
Thanks