OK first swing the sword once then press x only once,right after that you keep swinging until after 3rd swing.I am not a good instructer but will update this if any errors.
(OK this what it would look like:Swing sheild swing swing swing.The 3 swings should be not be interupted.)
Tips:when pressing x just press it very quickly and do not let sheild come up.
Sealed sword only has a 2-swing combo. The 3rd swing you saw was just a graphical glitch. It does no extra damage. You can get 4 swings out of a Calibur too, again the last swing is a graphical glitch. You don't even need to shield, just click repeatedly with a little bit of lag will do it.
It's not purely a graphical glitch, as it does only do three first swings while dealing three first swings' worth of damage. If it was just an animation error the third swing would do the full damage of the second combo swing. This "method" (See: bug) has been around for a long time and has been reported, though who knows if there are plans or even a way to fix it.
The best tactic for the big swords is a single attack and stopping the animation with a shield block, bar none. big damage, protection, and the speed of a 3-hitter.
And this does what exactly?