average the sum of every seventh number from 3 to a final value using for..Next loop

I wrote this code to find the average sum of every seventh number from 3 to whatever the user enters in the box -
The code I wrote doesn't give the correct average for example if user enter 20, the output should be average which for 20 is 3 because (3+10+17) / 3

Public Class Form1

    Private Sub AverageButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AverageButton.Click
        Dim inputString As String     'user input as string
        Dim inputInteger As Integer     'user input once convert
        Dim start As Integer = 3  'Answer   
        Dim countInteger As Integer
        Dim sum As Integer
        Dim average As Integer

        'Input

        inputString = Me.inputTextBox.Text

        'validate Input
        If inputString = "" Then
            MessageBox.Show("Please enter a number")    'If empty
            Exit Sub
        End If
        If inputString < 3 Then
            MessageBox.Show("Please enter a valid number above 3")  'if less than 3
            Exit Sub
        End If

        'Process
        inputInteger = Val(inputString)

        For count As Integer = start To inputString Step 7
            sum += count
            average = sum / count
        Next count

        'Output
        Me.outputLabel.Text = ("Average is " & average)

    End Sub
End Class

Can someone check this for me please?

I don't get it - I never thought I would be this dumb -
Can you just show it to me. the week past and I didn't get the point on it but I need to learn it.
Would do me a favor and correct the mistake that I have in that.