This is an icon by Russian monk Andrei Rublev, painted around 1435.
Rublev identified the three angels that visited Abraham and Sarah, with the Holy Trinity.
So, he took an icon/image of the Old Testament and used that same image in order to express another spiritual reality: the Holy Trinity.
God the Father is the on the left, God the Son is on the middle and the Holy Spirit on the right. Both the Son and the Holy Spirit are looking at the Father. Above each of them, lies a specific item: house which symbolizes the creation of the world (for the Father), tree which symbolizes the cross (for the Son) and a wave-like-rock (for the Holy Spirit); this wave symbolizes spiritual ascension and perfection that Christians are called to acquire.
They are sitting around what appears to be a table, but is in fact the altar upon which the sacrifice would be brought. On the altar there is the chalice with the Holy Body and Blood of Jesus Christ and The Son points towards it, looking at his Father; this means that the Son receives the ...hmm... i don't know how to call it....challenge (?) and proposition at the same time, to incarnate, live and save the world (but in the conditions created by the fall of Adam; the only difference is that Jesus Christ is sinless). This chalice is mentioned in Jesus' prayer in the Garden of Ghetsimani, and this chalice is put on the altar once the crucifixion is done (in the interval between burial and resurrection, the ManGod Jesus Christ is with his body on earth, in the tomb, and with his Spirit in Heaven, presenting the chalice/sacrifice to His Father and to the Holy Spirit.
