Due to the wording of this monster's effect, it is best to have as many different types of Lightsworns in your Deck as you can - though you do not necessarily need to have three copies of each.
It's possible to increase "Gragonith's" ATK/DEF to 7400/7000 (18 different Lightsworn monsters in your Graveyard, including another "Gragonith").
"Gragonith's" attack boost can be further amplified by "Realm of Light", which will add its Shine Counter boost on top of "Gragonith's" effects.
A "Gragonith" or two can be a great help during game 2 or 3 in a Match, as it's one of two Lightsworn monsters that is almost unaffected by "Light-Imprisoning Mirror", due to all but the End Phase milling effect being continuous effects.
Since "Judgment Dragon" needs 4 or more different Lightsworns monsters in your Graveyard to be summoned, it means that when you'll be able to summon your "Judgment Dragon", "Gragonith's" ATK would be at very least 3200. So you can summon "Judgment Dragon", use its effect to clear the field, then tribute it to summon "Gragonith", that would be more powerful in terms of ATK, and direct attack your opponent for heavy damage. Just do this if your opponent would be killed with that direct attack, since tributing "JD" looks quite like a crazy thing to do.