The　topologically　close-packed　phase　formation　is　heavily　influenced　by　the　concentrations　of　Co,　Ru,　and　Cr　in　Ni-based　superaloys.　For　this　purpose,　we　investigate　the　partitioning　behaviour　and　site　preference　of　these　additions　in　the　model　Ni-Al-X　single　crystal　superalloys　by　atom　probe　tomography　and　first-principles　calculations.　Compared　with　the　commercial　multicomponent　superalloys,　Co　and　Ru　still　strongly　partition　to　the　gamma　phase,　while　Cr　weakly　partitions　to　the　gamma　phase　in　the　ternary　model　alloys.　Based　on　the　gamma＇　phase　composition　analyses,　Ru　and　Cr　atoms　preferentially　substitute　for　the　Al　sublattice　sites,　whereas　Co　atoms　substitute　at　both　Al　and　Ni　sublattice　sites　in　the　ordered　gamma＇-Ni3Al　phase.　First-principles　calculations　on　the　substitutional　formation　energies　at　0　K　were　performed　to　understand　the　site　preference,　and　the　partitioning　coefficient　of　additions　was　given　in　a　quantitative　model.　The　partitioning　behaviour　and　site　preference　of　Co,　Ru,　and　Cr　by　theoretical　calculations　are　consistent　with　the　experimental　results.