The element that forms a 3 plus ion with the electron configuration of Kr 4d6 is Ruthenium (Ru).
To explain further, the electron configuration Kr 4d6 indicates that we have a total of 6 electrons in the 4d subshell, following the configuration of Krypton which accounts for the first 36 electrons. Ruthenium is located in group 8 of the periodic table and typically exhibits a +3 oxidation state among other states. This means it can lose three electrons, often from its 5s and 4d orbitals, to achieve a stable electron configuration.
When Ruthenium loses three electrons to form Ru3+, it results in an electron configuration that corresponds to 4d6, while the noble gas configuration of Krypton (Kr) is a significant indicator of the stability of this ionization state.