Autofill upwards!!!!!

I have a spreadsheet with 3 columns of data as follows

Col 1 Col 2 Col 3
Store Stock Item Sales
Item 1 22
Item 2 322
Item 3 16
Store 1 360
Item 1 11
Item 2 122
Item 3 46
Item 4 31
Store 2 210
etc

I have 2500+ rows of data so how can I easily get the store name to autofill upwards so it appears alongside each stock item, without copy/paste or dragging each one individually, so that I can then run a pivot table on the data

Note that each store does not necessarily carry the same number of stock items

Thanks in advance

Paul