Quick data manipulation script

Estimated duration: 1 day
Description: I have a spreadsheet in the following format.

SKU, Parent SKU, Associated Products
3100S, 3100, blank
3100M, 3100, blank
3100L, 3100, blank
3100, blank, blank

What I need is the data to be in the following format:

SKU, Parent SKU, Associated Products
3100S, 3100, blank
3100M, 3100, blank
3100L, 3100, blank
3100, blank, "3100S,3100M,3100L"

That is, the spreadsheet is currently set up with all the sub skus having a column to say what their parent sku is.

The way we need the data is that all parent skus have a column in which a list of comma separated subsku values appears.

This will need some kind of script or query run on it. I don't mind what language or how it is done.