It is only not less swingy for tasks that succeed or fail around the median. For tasks that one has a good chance pf success at (say need a 5 or higher), 3d6 gives a far greater chance of success than a d20, and for difficult tasks (require a 15 or higher), have a far less chance of success.

It is these outliers that have designers cry “too swingy” for d20s. Why would your character who has trained for years, have a 25% chance of failure at getting a 6, and a neophyte still has only a 50% of failure at the same task?