I'm sure they don't want to remind anyone of that terrible remake.
Funny enough Sword of Mana in of itself is the only game in the entire series which title is closest to the actual translation of the Japanese titling of the series.
Seiken Densetsu basically translates to Holy Sword Legend / Legend of the Holy Sword.
Of course due to Nintendo standards at the time, having the word "Holy" in the game title wasn't going to happen, due to their efforts to remove any references to religious connotation, among other things.
So it makes you wonder why SE didn't just simply swap Holy with another word...like Mana in this case.
Although plotwise for SD3 specifically. It deals with 8 ancient beings called the Mana Beasts which the Goddess used the Sword of Mana to defeat and seal them into the 8 Mana Stones, with the last of her power she became the Tree of Mana.
Two warring countries want the power of the 8 Mana Stones for themselves and seek to gain entrance into the Mana Holyland where the Tree and Sword of Mana itself resides. The goddess herself is losing power, so she sends one of her fairies to seek a hero to claim the Sword before it is too late, because if the Goddess / Tree of Mana dies before a Hero claims the Sword, the Mana Beasts will be freed from their sealed prison and destroy the world.
I guess the most plot propriate title for SD3 would probably be War of Mana due to the two Kingdoms at conflict trying to claim the power of the Mana Stones and Sword for themselves.