Final attempt.... HELP what am i doing wrong?

Option Strict On
Public Class HighestLowest
 
 
    ' code pseoudo
    ' INPUT 3 numbers
    ' pass to function lowest
    ' return lowest DISPLAY lowest
    ' pass to function highest
    ' return highest DISPLAY highest
 
    ' global vars
    Dim oneNum As Double = 0
    Dim twoNum As Double = 0
    Dim threeNum As Double = 0
    Dim highNum As Double
    Dim lowNum As Double
 
    Private Sub submitBtn_Click(sender As Object, e As EventArgs) Handles submitBtn.Click
 
        ' verify that the input is a number
        If IsNumeric(input1Tbx.Text) = True Then
            oneNum = CDbl(input1Tbx.Text)
       ElseIf : MessageBox.Show("Please enter numbers only.", "Please check your input values.", MessageBoxButtons.OK, MessageBoxIcon.Error)
            input1Tbx.Text = ""
            input2Tbx.Text = ""
            input3Tbx.Text = ""
            input1Tbx.Focus()
            Exit Sub
            If IsNumeric(input2Tbx.Text) = True Then
                twoNum = CDbl(input2Tbx.Text)
            ElseIf : MessageBox.Show("Please enter numbers only.", "Please check your input values.", MessageBoxButtons.OK, MessageBoxIcon.Error)
                input1Tbx.Text = ""
                input2Tbx.Text = ""
                input3Tbx.Text = ""
                input1Tbx.Focus()
                Exit Sub
                If IsNumeric(input3Tbx.Text) = True Then
                    threeNum = CDbl(input3Tbx.Text)
                ElseIf : MessageBox.Show("Please enter numbers only.", "Please check your input values.", MessageBoxButtons.OK, MessageBoxIcon.Error)
                    input1Tbx.Text = ""
                    input2Tbx.Text = ""
                    input3Tbx.Text = ""
                    input1Tbx.Focus()
                    Exit Sub
                End If
            End If
        End If
 
 
        highNum = largest(oneNum, twoNum, threeNum)
        largestLbl.Text = highNum.ToString
 
        lowNum = smallest(oneNum, twoNum, threeNum)
        smallestLbl.Text = lowNum.ToString
 
 
    End Sub
 
    Function largest(ByVal one As Double, ByVal two As Double, ByVal three As Double) As Double
 
        Dim tempMax As Double
        Dim finalMax As Double
 
        tempMax = Math.Max(one, two)
        finalMax = Math.Max(tempMax, three)
 
        Return finalMax
 
    End Function ' Maximum
 
    Function smallest(ByVal one As Double, ByVal two As Double, ByVal three As Double) As Double
 
        Dim tempMin As Double
        Dim finalMin As Double
 
        tempMin = Math.Min(one, two)
        finalMin = Math.Min(tempMin, three)
 
        Return finalMin
 
    End Function ' Minimum
End Class