Adding Text After Text To Split Strings?

I have string which is:

document1.txtdocument2.txtdocument3.txt

What I would like to do is split it so it shows in a listbox as

document1.txt
document2.txt
document3.txt

I have come up with a solution which may work. Its preety over complicated too me but this is the only way I could think of?

What I need help on is a code so that after every ".txt" it will place a "/" so the string will look like this:

document1.txt/document2.txt/document3.txt/

Then I would use this code to split the string:

Dim I3() As String
I3 = I.Split("/")
For g = 0 To I3.Length - 1
ListBox1.Items.Add(I3(g))
Next