All the Talismans were in the possession of their respective Sailor Senshi in the manga. When the Holy Grail appeared, the three Talismans resonated with each other, allowing Sailor Moon to transform into Super Sailor Moon. However, the main function of the three Talismans was to call upon Sailor Saturn, as they did after the Silver Millennium fell and at the end of the Infinity arc. Sailor Pluto also used the Garnet Orb to generate the Garnet Ball or attack enemies with Chronos Typhoon.
Unlike the Deep Aqua Mirror and the Space Sword, which in the anime were sealed in the Pure Hearts of Sailor Neptune and Sailor Uranus, the Garnet Orb was simply in Pluto's possession. When combined with the two other Talismans, it produced the Holy Grail.
Sailor Pluto appeared to use the power of the Garnet Orb to separate the Talismans from the Heart Crystals in which they were hidden, which allowed Sailor Uranus and Sailor Neptune to live even after they had the Talismans extracted from their bodies. It did not serve any other specific purpose in the anime other than this and producing the Holy Grail.
The three Talismans were based on the three imperial treasures of Japan. The jewel was said to represent benevolence.