Im fairly sure this is what you want:
Dim sTemp As String = vbNullString
Dim sTemp2 As String = vbNullString
Dim iMaxLen As Integer = 4
Dim iPos As Integer = 0
sTemp = TextBox1.Text
If sTemp <> vbNullString Then
If sTemp.StartsWith("1") Then
iMaxLen = 4
Else
iMaxLen = 3
End If
If Len(sTemp) > iMaxLen Then
If InStr(sTemp, ".") > 0 And InStr(sTemp, ".") < iMaxLen Then
iMaxLen = iMaxLen + 1
sTemp = Microsoft.VisualBasic.Left$(sTemp, iMaxLen)
ElseIf InStr(sTemp, ".") > iMaxLen Then
sTemp2 = Microsoft.VisualBasic.Left$(sTemp, InStr(sTemp, "."))
sTemp = Microsoft.VisualBasic.Left$(sTemp2, iMaxLen)
For iPos = iMaxLen To Len(sTemp2)
sTemp = sTemp & "0"
Next
ElseIf InStr(sTemp, ".") = iMaxLen Then
sTemp = Microsoft.VisualBasic.Left$(sTemp, iMaxLen + 1)
End If
End If
TextBox1.Text = sTemp
End If