Sub fiboncal()
   lim = 20
   Result = 1

  fib0 = 0
  fib1 = 1

For i = 2 To lim
    fib = fib1 + fib0
    Result = Result + fib
    fib0 = fib1
    fib1 = fib
Next i

    MsgBox "Cows is " & Result
End Sub

Sub CowCal()
lim = 20
Result = 4
  fib0 = 1
  fib1 = 1
  fib2 = 2

For i = 4 To lim
    fib = fib2 + fib1 + fib0
    Result = Result + fib
    fib0 = fib1
    fib1 = fib2
    fib2 = fib
Next i

 MsgBox "Cows is " & Result
End Sub

Private Const YearCount As Long = 40
Private FNewBornCount As Long
Private FDeathCount As Long


Public Sub TestCows()
    FNewBornCount = 0
    FDeathCount = 0
    NextBorn 0
    Debug.Print "Всего родилось коров " & CStr(FNewBornCount)
    Debug.Print "Всего умерло коров " & CStr(FDeathCount)
    Debug.Print "Количество коров на текущий год " & CStr(FNewBornCount - FDeathCount)
End Sub

Private Sub NextBorn(ByVal itsYear As Long)
    FNewBornCount = FNewBornCount + 1
    If (itsYear + 3) <= YearCount Then NextBorn itsYear + 3
    If (itsYear + 5) <= YearCount Then NextBorn itsYear + 5
    If (itsYear + 7) <= YearCount Then NextBorn itsYear + 7
    If (itsYear + 8) <= YearCount Then FDeathCount = FDeathCount + 1
End Sub

Const limit = 40

Function countCows(age)
    Dim sum
    sum = 0
    If age + 3 <= limit Then sum = sum + 1 + countCows(age + 3)
    If age + 5 <= limit Then sum = sum + 1 + countCows(age + 5)
    If age + 7 <= limit Then sum = sum + 1 + countCows(age + 7)
    countCows = sum
End Function

Sub test()
    Debug.Print "Всего коров за период " & (1 + countCows(0))
End Sub

' ...
For i = itsYear + 3 To WorksheetFunction.Min(itsYear + 7, YearCount) Step 2
' ...