Sub Macro2()
'Graham Mayor - https://www.gmayor.com - Last updated - 20 Jan 2020 

 Dim i As Long
Dim oPara As Paragraph
Dim oRng As Range

    'Create styles or use existing styles
    On Error Resume Next
    ActiveDocument.Styles.Add Name:="Body_Spaced", Type:=wdStyleTypeParagraph
    ActiveDocument.Styles("Body_Spaced").AutomaticallyUpdate = False
    With ActiveDocument.Styles("Body_Spaced").Font
        .Name = "Calibri"
        .Size = 12
    End With
    With ActiveDocument.Styles("Body_Spaced").ParagraphFormat
        .SpaceAfter = 12
    End With
    ActiveDocument.Styles.Add Name:="Body_Unspaced", Type:= _
                              wdStyleTypeParagraph
    ActiveDocument.Styles("Body_Unspaced").AutomaticallyUpdate = False
    With ActiveDocument.Styles("Body_Unspaced").Font
        .Name = "Calibri"
        .Size = 12
    End With
    With ActiveDocument.Styles("Body_Unspaced").ParagraphFormat
        .SpaceAfter = 0
    End With
    On Error GoTo 0
    'end of style creation
    'apply styles
    For i = 1 To ActiveDocument.Paragraphs.Count
        Set oPara = ActiveDocument.Paragraphs(i)
        If i Mod 3 = 0 Then
            oPara.Range.Style = "Body_Spaced"
        Else
            oPara.Range.Style = "Body_Unspaced"
        End If
    Next i
    Set oRng = ActiveDocument.Range
    With oRng.Find
        .Style = "Body_Spaced"
        Do While .Execute
            If Len(oRng.Next.Paragraphs(1).Range) > 1 Then
                oRng.InsertAfter "+" & vbCr
            End If
            oRng.Style = "Body_Unspaced"
            oRng.Collapse 0
        Loop
    End With
    Set oRng = Nothing
    Set oPara = Nothing
End Sub

Sub Macro3()
'Graham Mayor - https://www.gmayor.com - Last updated - 20 Jan 2020 

 Dim oRng As Range
Dim lngPara As Long
Dim lngCount As Long
Const lngNum As Long = 3    'the number of paragraphs to group
    lngCount = ActiveDocument.Paragraphs.Count
    For lngPara = lngCount To 1 Step -1
        'eliminate empty paragraphs at the end from the count
        If Len(ActiveDocument.Paragraphs(lngPara).Range) = 1 Then
            lngCount = lngCount - 1
        Else
            Exit For
        End If
    Next lngPara
    'Find the end of the last of the grouped paragraphs
    Do Until lngCount Mod lngNum = 0
        lngCount = lngCount - 1
    Loop
    'Insert paragraphs containing a '+' character
    For lngPara = lngCount To lngNum Step -lngNum
        Set oRng = ActiveDocument.Paragraphs(lngPara).Range
        oRng.InsertAfter "+" & vbCr
    Next lngPara
    Set oRng = Nothing
 End Sub

oRng.InsertAfter "+" & vbCr