The Imperial Regalia of Japan, also known as the Three Sacred Treasures of Japan, consist of the sword Kusanagi no Tsurugi (草薙劍), the mirror Yata no Kagami (八咫鏡?), and the jewel Yasakani no Magatama (八尺瓊曲玉?). The regalia represent the three primary virtues: valor (the sword), wisdom (the mirror), and benevolence (the jewel).
According to legend, these treasures were brought by Ninigi-no-Mikoto, legendary ancestor of the Japanese imperial line, when his grandmother, the Sun Goddess Amaterasu, sent him to pacify Japan. Traditionally, they were a symbol of the emperor’s divinity as a descendant of Amaterasu, confirming his legitimacy as paramount ruler of Japan. When Amaterasu hid in a cave from her brother Susanoo, thus plunging the world in darkness, the goddess Ame-no-Uzume hung the mirror and jewels outside the cave and lured her out of the cave, at which point she saw her own reflection and was startled enough that the gods could pull her out of the cave. Susanoo later presented the sword Kusanagi to Amaterasu as a token of apology; he had obtained it from the body of an eight-headed serpent, Orochi. (via Wikipedia)
In Sailor Moon
In Sailor Moon, the talismans were three mythical items held by Sailor Uranus, Sailor Neptune, and Sailor Pluto. They were very powerful artifacts and acted as source of power for each respective Senshi. The three talismans were the Space Sword (Uranus), the Deep Aqua Mirror (Neptune), and the Garnet Orb (Pluto). In the anime, they were hidden in Pure Heart Crystals and the search for them was a major plot point during the first part of the S season. When brought together, they summoned the Holy Grail. In the manga, however, each Senshi was already in possession of her talisman at the start and bringing them together summoned Sailor Saturn instead of the Holy Grail.
Of course, the similarities are obvious given that the talismans were based on the Three Sacred Treasures. The Space Sword was based off of Kusanagi no Tsurugi, the Deep Aqua Mirror was the equivalent of Yata no Kagami, and the Garnet Orb was Yasakani no Magatama.
The attributes associated with the Sacred Treasures also seem to match with each Senshi. The sword represented valor, and Sailor Uranus has proven time and time again to be immensely courageous and fearless in the face of danger, most notably in battle. The trait associated with the mirror, wisdom, fits Sailor Neptune like a glove. She often used her mirror to see things that she cannot and to reveal truth. With wisdom comes the truth, and Neptune would often use this to her advantage in battle. And finally, the characteristic connected with the jewel, benevolence, perfectly fits Sailor Pluto. She acts as a loving protector to Chibiusa and tirelessly watches over the Door of Space-Time, ensuring that time remains safe and guarding it with her life. She also acts as a trusted ally to the Inner Senshi, even at times when her other companions didn’t.