Lake Cornelius and Lake Davidson are essentially arms of Lake Norman, connected by pipes under the interstate. You can't boat from one to the next, but water and fish can move between them.
Lake Norman (and by extension the other two lakes) is dominated by spotted bass in that section of the lake... Maybe 10 to 1 spot to LMB ratio in that part of the lake? Less the further up you get. And spotted bass do not get very large..if the 6lb fish you caught was a spot, it was pretty close to the state record (which will come out of that lake, I'm sure).
While the spots aren't as big as LMB, they generally fight harder.
Prior to the spots being introduced, Lake Norman had a reputation as being the "Dead Sea", or the lake with "Big homes but small fish". So the spots are welcome, IMO. I don't fish the riverine Mountain Island Lake very much, but I understand that spotted bass are the primary species there as well.
If you're looking for trophy sized fish, I'd say try the Yadkin chain. If you're Ok with bass in the 2-5lb range, then Lake Norman and the other two you mentioned are good places to try. I'm not saying that a trophy fish is impossible.... I'm just saying that the odds are way against finding one there.
It's a little easier accessing Davidson Lake... You can launch kayaks from the canoe trail in Davidson or Traco road. I don't think there's a public launch on Cornelius lake, but there are several in communities if you know someone who lives there.