How to use a loop through multiple similar textboxes?

If I understand you correctly, this code can be easily generalized by creating a Sub and calling it with your inches calculation and Label text,:oord are reachable (in scope) Functions:

Triangulate(inches(aft1.Text, ain1.Text), inches(bft1.Text, bin1.Text), 1)
Triangulate(inches(aft2.Text, ain2.Text), inches(bft2.Text, bin2.Text), 2)
Triangulate(inches(aft3.Text, ain3.Text), inches(bft3.Text, bin3.Text), 3)

EDIT-2

Although I have tested the following up to a point, since I do not have your chart properties nor the inches, xcoord and ycoord functions I could not test it completely, so try it out and let me know how it goes.

Sub TriangulateAll()
    Try

        Dim aft As New SortedList(Of String, TextBox)
        Dim ain As New SortedList(Of String, TextBox)
        Dim bft As New SortedList(Of String, TextBox)
        Dim bin As New SortedList(Of String, TextBox)

        For Each ctl As Control In Controls
            If TypeOf (ctl) Is TextBox Then
                Select Case ctl.Name.Substring(0, 3)
                    Case "aft"
                        aft.Add(ctl.Name, ctl)
                    Case "ain"
                        ain.Add(ctl.Name, ctl)
                    Case "bft"
                        bft.Add(ctl.Name, ctl)
                    Case "bin"
                        bin.Add(ctl.Name, ctl)
                End Select
            End If
        Next
        Dim a As New List(Of Double)
        Dim b As New List(Of Double)

        For Each kvp_aft As KeyValuePair(Of String, TextBox) In aft
            For Each kvp_ain As KeyValuePair(Of String, TextBox) In ain
                a.Add(inches(kvp_aft.Value.Text, kvp_ain.Value.Text))
            Next
        Next
        For Each kvp_bft As KeyValuePair(Of String, TextBox) In bft
            For Each kvp_bin As KeyValuePair(Of String, TextBox) In bin
                b.Add(inches(kvp_bft.Value.Text, kvp_bin.Value.Text))
            Next
        Next
        For i As Int16 = 0 To aft.Count - 1
            Triangulate(a(i), b(i), i.ToString())
        Next
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub

EDIT Replace the 3 calls to Triangulate with the following code to completely generalize the Triangulation routine. Note that this routine expects there to be an equal number of each of the TextBox controls: aft, ain, bft, bin. I haven't tested this but it should work.

    Dim aft As List(Of TextBox) = Nothing
    Dim ain As List(Of TextBox) = Nothing
    Dim bft As List(Of TextBox) = Nothing
    Dim bin As List(Of TextBox) = Nothing

    For Each ctl As Control In Controls
        If TypeOf (ctl) Is TextBox Then
            Select Case ctl.Name.Substring(0, 2)
                Case "aft"
                    aft.Add(ctl)
                Case "ain"
                    ain.Add(ctl)
                Case "bft"
                    bft.Add(ctl)
                Case "bin"
                    bin.Add(ctl)
            End Select
        End If
    Next
    aft.Sort()
    ain.Sort()
    bft.Sort()
    bin.Sort()

    For i As Int16 = 0 To aft.Count - 1
        Dim a As Double = inches(aft.Item(i).Text, ain.Item(i).Text)
        Dim b As Double = inches(bft.Item(i).Text, bin.Item(i).Text)
        Triangulate(a, b, i.ToString())
    Next

   Sub Triangulate(a As Double, b As Double, LabelValue As String)
       If a <> 0 And b <> 0 Then
           Dim targetpoint As Int32
           targetpoint = Chart1.Series("Drawing").Points.AddXY((xcoord(a, b)), ((ycoord(a, b))))
           Chart1.Series("Drawing").Points.Item(targetpoint).Label = LabelValue
       End If
   End Sub