If you want to see the icon in person, then go to the Tretyakov Gallery in Moscow. Here is their description of the icon:
Holy Trinity (Troitsa)
141.5 x 114
The icon of the “Holy Trinity” is the most famous work of the brilliant Russian artist Andrei Rublev. According to the testimony of one of the 17th century sources, it was painted “in praise of Sergii Radonezhsky” at the order of his pupil and successor Abbot Nikon. At the basis of the iconography is the Biblical tale (Genesis 18) of the appearance to saint Abraham of God in the form of three angels. Abraham and his wife Sarah entertained the three angels in the shade of an oak when Abraham understood that the angels were the embodiment of God in three faces. Avoiding details which were customary in the subject of the “Hospitality of Abraham”, Andrei Rublev achieved extraordinary symbolic profundity in his work. In Rublev’s icon all attention is concentrated on the three angels and their silent exchange. They are depicted as seated around an altar in the center of which there is a chalice of the Eucharist with the head of a sacrificial calf which symbolizes the lamb of the New Testament, i.e., Christ. The left and centre angels bless the chalice. God the Father blesses God the Son for death on the cross in the name of love for people. God the Holy Spirit (the right angel) is present here to provide comfort, confirming the high logic of sacrificial, all-forgiving love. The content of the “Holy Trinity” is ambiguous. The monument is multi-faceted in its themes. Firstly, it embodies the idea of the triune Divinity. During the times of Sergii Radonezhsky and Andrei Rublev, the subject of the Trinity was understood as a symbol of spiritual unity, mutual love, the world and readiness to sacrifice oneself.
Wellsprings is an English Catholic site which contains several unique and valuable resources including an explanation of the symbolism of the Rublev Icon.
Soul Shepherding blog has a useful explanation of the icon.
Ann Persson discusses her experiences using the Rublev Icon as a tool for Christian meditation. Part of the St Paul’s Forum monthly series that highlights contemporary Christian authors and their work.
A Reader’s Guide to Orthodox Icons Who’s Who in the Trinity Icon?
Christopher Evan Longhurst Andrei Rublev’s Trinity Icon (from Obbservatore Romano)
James C. Harris M.D. Andrei Rublev’s Old Testament Trinity (Journal of the American Psychiatric Association)