Ok , first i was wondering if I really need three way swivels or can i tie two lines on one hole ? And I was wondering what rigs are best for: flounder ? Black drum? Redfish? And specked trout?

You can tie two lines in one hole on a swivel if you like untangling lines more than actually fishing. There is a reason these things were invented. Even with a three way, you will probably get tangles, just not as many or as bad.

Drum rig
rod length +3' of min. 50# shockleader with a sinker slide on it----swivel---3" of 80# fluorocarbon w/ a 9/0 circle hook. this rig works for red drum (redfish) and black drum you just use different bait. the reason for the short bite section is to keep the rig from "hellocoptering" (s) so casting distance is maximized, sometimes 10 more yards is the difference between catching fish and not.
for flounder and commercial flounder rig or a river rig what matters most is your bait and where ya put it.
specs, popping cork with 2' of flouro below it with a 4/0 circle baited with live shrimp works as good as anything.
RR

Just tie a hi and lo rig. If your using a spro jig tie it on the bottom . If no jigs tie a loop on the bottom for sinker and 2 dropper loops 1 12-18" above and the other 36" from the bottom , attach teasers or hooks.