searrch and combine using text file

I have the following question. I have a text file (shown below) and it contains a couple of lines, where each line is separated in 3 parts with a colon. What I would like to do is search the first part of the lines for specific content and use only those lines to calculate the last part of the lines.
1:test1:3
2:test2:5
3:test3:6
4:test4:9
5:test5:8
6:test6:7
For example lines with in the first part 1,3 and 6, calculate the total of the third part of these lines,that would be 16. The same for 2,4 and 5 tahta would be 22. The code I used so far:

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim AllText As String = "", LineOfText As String = ""
        Dim split(2) As String

        OpenFileDialog1.Filter = "Text files (*.TXT)|*.TXT"
        OpenFileDialog1.ShowDialog() 'display Open dialog box
        If OpenFileDialog1.FileName <> "" Then
            Try 'open file and trap any errors using handler
                FileOpen(1, OpenFileDialog1.FileName, OpenMode.Input)
                Do Until EOF(1) 'read lines from file
                    LineOfText = LineInput(1)
                    split = LineOfText.Split(":")
                    'add each line to the AllText variable
                    AllText = AllText & split(0) & "     " & split(1) & "     " & split(2) & vbCrLf
                Loop
               TextBox1.Text = AllText 'display file
            Catch
                MsgBox("Error opening file.")
            Finally
                FileClose(1) 'close file
            End Try
        End If
    End Sub

End Class