VS 2005 : Create A Source To Get The Strings From The Text File Using ReadLine Method?

I need a bit of advice. I am working on my project with the listbox. I know how to create a source to get the strings from the text file using ReadLine method, but I have no idea how to split the strings in the text file by on the third commas and ignore the other strings while select with the matched strings on the listbox for each row.Here it is an example listbox Here it is an example items on the listbox

random item 1
random item 2
random item 3
random item 4
random item 5

And here it is an example text file.

example strings one, any strings 1, any random strings 1, other strings 1, final end of strings 1
example strings two, any strings 2, any random strings 2, other strings 2, final end of strings 2
example strings three, any strings 3, any random strings 3, other strings 3, final end of strings 3
example strings four, any strings 4, any random strings 4, other strings 4, final end of strings 4
example strings five, any strings 5, any random strings 5, other strings 5, final end of strings 5

The listbox is display the list of an example items, so if I select the "random item 1" item on the listbox, it read the strings through in the text file on the first line to get the strings on the third commas in the same line which it is (any random strings 1) to split it while ignore the other strings (other strings 1, final end of strings 1) that come fourth and fifth commas. So, if I select the "random item 2" item on the listbox, it read the the strings through in the text file on the second line to get the strings on the third commas in the same line which it is (any random strings 2) to split it while ignore the other strings (other strings 2, final end of strings 2) that come fourth and fifth commas and so on...That is what I am trying to achieve by select the each listbox item to get the correct strings for each line in the text file that come on the third commas while to ignore the other strings.