Like everyone else said, I run floating plugs on three-ways with about ten inches of dropper, and three to four feet of line on the lure.
I keep plenty of Floating Rapalas and Storm Thundersticks on hand in chartreuse, firetiger, orange, silver, and blue, with some other makes and colors thrown in to be safe. Rapalas have a tight swimming action, and Thundersticks wobble a lot more, and sometimes the fish definitely prefer one action over the other, just as they usually prefer one color over another.
If you run live-bait rigs such as crawler harnesses that use spinner blades on three ways, you want to have less line connecting the bait to the swivel. I generally run about 18″ of line on my crawler harnesses, which I make myself. The reason for this is that the more line you have on your bait harness, the lower it runs. The spinner is there to attract fish, but it also lifts the bait off the bottom. If you try running three or four feet of line on a crawler harness, the action of the spinner will not lift it off the bottom, and your bait will drag in the mud, rock, or sand, and be far less effective.
Crawlers are a summer bait for walleye, and generally do not work when the water temps are below about sixty degrees. Leeches have their moments, but generally do not work as well for walleye on the Mississippi as crawlers do, in my experience.