Copy 1 cell formula to a range of cells and loop every 3 rows

Here are the individual lines of code. How does one create a macro loop instead of all the lines of code:

Range("D21").Select 'copy Subject
Selection.Copy
Range("E21:O21").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D24").Select 'copy Next Subject
Selection.Copy
Range("E24:O24").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D27").Select 'copy Next Subject
Selection.Copy
Range("E27:O27").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D30").Select 'copy Next Subject
Selection.Copy
Range("E30:O30").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D33").Select 'copy Next Subject
Selection.Copy
Range("E33:O33").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D36").Select 'copy Next Subject
Selection.Copy
Range("E36:O36").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D39").Select 'copy Next Subject
Selection.Copy
Range("E39:O39").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D42").Select 'copy Next Subject
Selection.Copy
Range("E42:O42").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D45").Select 'copy Next Subject
Selection.Copy
Range("E45:O45").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

Range("D48").Select 'copy Next Subject
Selection.Copy
Range("E48:O48").Select
Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False

I'm not too sure about the cell references. This must take place in the active worksheet of a workbook that has 67 worksheets. I can loop through the worksheets but this has me a bit stuck.

Thanks in advance.

for count = 21 to 48 step 3
Range("D" & Count).copy Range("E" & count & ":O" & count)
next

?

Thanks, that worked!