It is bad practice to insert empty paragraphs. Word is not a typewriter, which is the origin of inserting space using empty paragraphs, so spacing should be applied with styles. You probably have existing styles with space that would so the job, but the following will create styles and add them as appropriate to give the required spacing. If you have suitable styles, remove the style creation and apply them instead.
Code:
Sub Macro1()
Dim i As Long
Dim oPara As Paragraph
'Create styles or use existing styles
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
'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 oPara = Nothing
End Sub