screws nin end grain...
Screwing into end grain from either a top or a side panel is risky because the plys can separate and the screws loose their grip. However, if you but a cleat below the stack of three cabinets/boxes you can reduce the load hanging from the French cleat. I disagree with Steve, because they are 3 separate units. If they were all one, backed with one sheet of ply, then yes, it will support it.
The cleat underneath will support most of the weigh, so use long screws into the studs. :smile3:
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)