Actually, the keene a52, which is a 10" wide sluice, will handle a 3" suction nozzle feed. As hoser said, convert to dredge riffles! More water/larger rocks make the change necessary.
Want proof that it can handle a 3"?
here ya go
There are some modifications to do, but it can and will handle it if you do your research and your testing.....
good luck