Several slick methods here. I thought yours was the simplest, cleanest. I
shortened it even more and it ran in about one second.

Application.ScreenUpdating = False
Range("A1").Activate

For i = 65 To 90
For j = 65 To 90
For k = 65 To 90
ActiveCell = Chr(i) & Chr(j) & Chr(k)
ActiveCell.Offset(1, 0).Activate
Next k
Next j
Next i

Application.ScreenUpdating = True


"Gary''s Student" wrote:

Sub ThreeLetter()
l = 1
For i = 65 To 90
x = Chr(i)
For j = 65 To 90
y = Chr(j)
For k = 65 To 90
z = Chr(k)
Cells(l, 1).Value = x & y & z
l = l + 1
Next
Next
Next
End Sub

--
Gary''s Student - gsnu200777