I inherited an Excel spreadsheet where one column on each
row has a list box in each cell. I want to add 3 more
items to the list, but cannot figure out how to do this.
I have looked at the properties of the list box but see no
reference to the range of cells that the list populates from.

With the Listbox highlighted, double click it. If it is from the Forms
toolbar, a form will appear where you can enter the Input Range.
If it is from the Control Toolbox, fill in the ListFillRange property.

HTH,
Merjet