当初の希望図に合わせるなら・・
Sub test()
Dim sh As Worksheet
Dim Er, Col, k As Long, j As Long
Dim sum計画, sum実績, sum遅延
Dim rsltLine()
Application.ScreenUpdating = False
Set sh = ThisWorkbook.Sheets("main")
Er = sh.Cells(Rows.Count, 1).End(xlUp).Row '最終行
Col = sh.Cells(1, 2).End(xlToRight).Column '最終列
ReDim resltLine(1 To 1, 3 To Col)
For k = 4 To Er Step 3
sum計画 = Empty
sum実績 = Empty
sum遅延 = Empty
For j = 3 To Col
sum計画 = sum計画 + sh.Cells(k + 1, j)
sum実績 = sum実績 + sh.Cells(k + 2, j)
sum遅延 = sum計画 - sum実績 - sh.Cells(k + 1, j) '当日計画値を引けるだけ引く
resltLine(1, j) = IIf(sum遅延 <= 0, Empty, sum遅延)
Next j
sh.Cells(k, "C").Resize(1, Col - 2) = resltLine
Next k
Application.ScreenUpdating = True
End Sub