I can only see a 3 way rig working like you describe either if you're in current or fishing straight down. Bank fishing shallow water, wouldn't your bait still be laying on bottom due to the angle of your line..?
Alot of bank anglers don't understand this for some reason. But you are 100% correct, that from the bank when using a 3 way rig you really don't get your bait off the ground that much unless you are in fairly deep water. (Or deep water right close to shore where you don't cast out that far.) I did a test once where I casted out very far and didn't realize the water was not that deep, had someone in a boat check out my line and bait and sure enough, my bait was on the bottom still. lol... And then even in deep water, depending on the angle of your line, you may only be inches off the bottom. If the bottom is clear, sandy, etc, you're not going to get that much of an advantage using a 3-way imop. And if the bottom has a ton of sludge, plant life, etc, you're probably better off using a float anyway from the bank to keep the bait well above all that.
Now..I DO use a 3-way when I'm fishing a dam or in a situation where I'm casting out and reeling in, etc. Fishing for stripers for sure I'll use this method. The bait starts out near or just off the bottom, but as I reel in the angle starts to get more vertical allowing the bait to come off the bottom more. I'll also use a much longer leader for the weight in this scenario, 3-6ft which gets the bait way off the bottom as it gets closer.