If you are after big flatheads, you need to use live bait, because they are very preditory. I catch them on large perch (sunfish) and seldom are they caught on the bottom. They're mouth is designed to strike a bait from underneath it, so floating a large live bait works well.
For big blues shad are hard to beat. They do not need to be alive, but they usually need to be quite fresh, so keep them on ice. Sometimes whole shad work, sometimes the head works, sometimes they want a shad fillet. Drifting the bait near the bottom is affective, but sometimes blues can be caught under water turkey roosts(cormorants) right on the surface.
Channels will eat most anything, but for big channels shad and perch work well....avoid those commercial baits....they are for the most part only good for small fish. Natural baits work best. Crawfish, earthworms, grasshoppers, even minnows as well as the shad and perch. Many times folks catch good channels on cut up carp. I have had luck with bloodbait, but it's hard to find now days....made from chicken blood.