please use this code this will solve your prob :)
Dim str As String
Dim strArr() As String
Dim count As Integer
str = txtUsername.Text ' txtbox having txt you wana split.
strArr = str.Split("_")
For count = 0 To strArr.Length - 1
If txt1.Text = "" Then
txt1.Text = strArr(count).ToString
If txt2.Text = "" Then
txt2.Text = strArr(count).ToString
If txt3.Text = "" Then
txt3.Text = strArr(count).ToString
if your prob is solved then pleas mark this thread solved and also dont forget to add reputation .