Looks like you are writing VB.Net code so the stuff thus far does not really apply
Is that the only line you have or do you have more?
What method are you using to open the file?
The true flag on the first line above tells the stream writer to create the file if it does not exist or if it does exist that it should append to the end of the existing file.
Dim OutFile as New System.IO.StreamWriter("c:\" & textbox1.Text & TextBox2.Text &".csv",True)
OutFile.Writeline(Textbox1.Text & "," & Textbox2.Text & "," & Textbox3.Text)