The expression "the tree of three borders" most often refers to a commemorative monument located in the European Monument, which marks the tripoint between France, Germany, and Luxembourg. It is not a tree in the strict sense, but rather a set of symbolic stone blocks.
The monument was erected at the exact spot where the borders of France, Germany, and Luxembourg meet, a strategic location symbolizing a united Europe.
The location, and the monument placed there, was chosen for its significance as a convergence of several nations. It was placed in the European Monument Park, which is built on this tripoint.
The establishment of this monument is part of a larger project to commemorate European unity, and its geographical position is its main symbolic element.