With only 3 fish feeding twice a day will almost definitely mean still over feeding. I feed my fish about once every 3 days, and I've got 28 of them. And I still have loads of snails as well. I go around and squash all visible ones whenever I can be bothered, but that's more an attempt to keep them under control a bit rather than get rid of them outright.
To the OP: unfortunately, there is little that can be done to really erradicate them. Especially with a tank like yours. A different tank may help the problem by leaving less places for the snails to breed and food to lodge, but they will always find a way. That's how nature works. If you want to get rid completely then buy a new tank, cycle it, move your fish over and completely break down the old tank. If you don't take anything but fish from old to new tank you won't have snails. Otherwise you'll just have to live with them.