Cessnas and pre-NG 737s both had slotted flaps. Cessnas are single slotted, Boeings are up to Triple slotted. As long as there's a slot somewhere.
Its a question of efficiency vs. weight/complexity.
Multiple slotted flaps are traditionally more efficient, but heavy and complicated. Apparently Boeing now have found a double slotted design for the NG
737s which does as well as the original triple slotted types.
The 747SP used a single slotted flap, the other versions used a triple. Less weight, less drag through not needing such large track fairings, but I believe I'm correct in saying it wasn't as efficient. It didn't need to be however, since the same wing (dimensionally) was lifting a much smaller airplane, and reasonable take-off/approach speeds were attainable.
Incidentally the SP
flap translation path was interesting. The flap leading edge didn't simply move aft along a track like a Cessna while the trailing edge moved aft/down. The LE moved aft and down on a pivot arm, so it was significantly below the undersurface of the wing. This I recall from the diagrams in a South African Airways 747SP differences manual I read on the flight deck of one over the Indian Ocean, and look forward to a more accurate description from someone who knows what he/she is talking about!!
Regards - Musang