Compile Error when using different VBA codes in same window - excel

Question

I have a number of things I want to achieve using VBA on a particular sheet.
1) Have a 3 button message box pop up when a certain condition is met.
2) Display the active cell address in a specific cell.
3) When hitting Enter after inputting data only to empty cells in a particular column, make the cursor jump to another column on the same row.
(Codes for each of these are at the end of the post)
I have the code to do all three of these things and they all work fine on their own, indeed the codes for items 1 & 2 also both work fine together, but when I add the code for item 3, a message box appears with
"Compile Error:
Ambiguous name detected: Worksheet_SelectionChange"
and the offending article in the code window is also highlighted.
I've noticed that the code for items 2 & 3 have the heading "Private Sub Worksheet_SelectionChange(ByVal Target As Range)"
If I remove the code for item 2, im then faced with a different message box
"Compile Error: Only comments may appear after End Sub, End Function, or End Property"
and highlights the words "Option Explicit" in the code window.
This also seems to break the code for item 1 as well.
I suspect the fact that I have two sets of code in the same window with the same "heading" might be the issue here. Is there a way for me to "blend" them so that they'll all play nicely with each other?
**---CODE FOR ITEM 1---**
Private Sub Worksheet_Calculate()
Dim r As Range
For Each r In Range("L:L")
If r.Value < 0 Then
result = MsgBox("You do not have enough stock to fulfil this request" & vbNewLine & vbNewLine & vbNewLine & _
"Please click: -" & vbNewLine & vbNewLine & _
" -Abort to order more stock" & vbNewLine & _
" -Retry to enter a different value" & vbNewLine & _
" -Ignore to receive stock", _
_
vbAbortRetryIgnore + vbDefaultButton2 + vbExclamation, "Negative Stock Level Warning")
End If
Next r
If result = vbAbort Then
MsgBox "Opening web browser", vbOKOnly + vbInformation, "New program warning!"
ActiveWorkbook.FollowHyperlink _
Address:="https://uk.rs-online.com/login", _
NewWindow:=True
End If
If result = vbRetry Then
MsgBox "Please enter a smaller parts count value", vbOKOnly + vbInformation, "Parts Count Input"
ActiveCell.Offset(-1, 0).Select
End If
If result = vbIgnore Then
MsgBox "You will now be directed to the Goods In window", vbOKOnly + vbInformation, "Receive Stock"
Sheets("Goods In").Activate
End If
End Sub
---CODE FOR ITEM 2---
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Range("A1048575") = ActiveCell.Address
End Sub
---CODE FOR ITEM 3---
Option Explicit
Dim emptyCell As Boolean
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.CountLarge > 1 Then Exit Sub ' don't bother with multicell selections
If Cells(1, Target.Column).Value <> "S.I.#" Then Exit Sub ' don't bother with selections outside "S.I.#" column
If emptyCell And Not IsEmpty(Target.Value) Then Cells(Target.Row, Range("A1", Cells(1, Columns.Count).End(xlToLeft)).Find(what:="Count", _
LookIn:=xlValues, lookat:=xlWhole).Column).Select ' if current cell was empty and now it is not then skip to "Count" column same row
End Sub
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
emptyCell = IsEmpty(Target.Value) ' store the info about current selection being empty
End Sub