Here in the U.S., there were two Three Investigators boxed sets released by Random House. Boxed Set #1 was definitely available in retail shops as early as Christmas 1978. This set contains titles #1, 7, 8, and 10 in paperback with Stephen Marchesi's cover art. Later issues of this set will contain book #1 with Adragna cover art and, less commonly, #10 with Adragna cover art. The box has a background color of dark blue, has the wording: "Alfred Hitchcock and the Thee Investigators", and features Marchesi artwork from "Silver Spider" on one side and "Terror Castle" on the other. The back, or spine of the box, has Hitchcock's silhouette and then lists the titles of the four books. Early sets have a printed price of $6.00 on the box spine while later sets do not.
Boxed set #2 contains the following four titles in paperback: #3, 26, 27, and 37. All four books feature Adragna's cover art, #'s 3, 26, and 27 are Hitchcock Cover paperbacks and #37, obviously, is a Keyhole Cover paperback. Random House was possibly overstocked on the three Hitchcock Cover books and was attempting to sell off as many of them as quickly as possible since all of the first 30 titles were converted to the Keyhole Cover edition in 1984 and 1985. The box is the same as that used for set #1 but it has a paste-over on the spine that lists the four titles in the box and there is no Hitchcock silhouette on the spine. Set #2 was available in 1984 and probably not for very long.
Set #1 is fairly easy to acquire but Set #2 is much more difficult to find. Boxed sets which are unopened and still encased in the original shrink wrap are rare. Check out the scan below of Boxed Set #1 in mint, unopened condition from 1978 (books have no # on the spines and the box has the $6.00 printed price).