Go to the Properties page for the ListBox, click on the 'Column Headers' tab, and set the index to whichever column you don't need, and click 'Remove Column'.

You must have inadvertantly added 3 columns - as far as I'm aware, you can't add columns at run-time.

You can change a column's width by putting a higher number in the 'width' box (also on the 'Column Headers' tab).