We report on the magnetic and electronic properties of ${\mathrm{NpSi}}_3$, ${\mathrm{NpGa}}_3$, and ${\mathrm{NpIn}}_3$, which crystallize in the cubic ${\mathrm{AuCu}}_3$ crystallographic structure (space group Pm3m). Studies by Mössbauer as well as ac and dc magnetization techniques are presented. From the Mössbauer isomer-shift data we argue that the Np ion in Np${\mathit{X}}_3$ is close to the formal 3+ ${(}^5$${\mathit{I}}_4$) charge state. The magnetic moment of the Np in ${\mathrm{NpSi}}_3$ is totally suppressed, whereas in ${\mathrm{NpGa}}_3$ a localized (narrow band) moment (1.5${\mathrm{\mu }}_{\mathit{B}}$) is established. However, in ${\mathrm{NpIn}}_3$ at 4 K, a modulated magnetic moment [(0–1.5)${\mathrm{\mu }}_{\mathit{B}}$] is observed. Comparing the magnetic behavior of the Np${\mathit{X}}_3$ family (X=Si, Al, Ga, Ge, Sn, and In), we find an impressive variation of the magnetic properties, from temperature-independent paramagnetism, localized and modulated ordered moments, to the formation of a concentrated Kondo lattice. Hybridization of 5f electrons with ligand electrons appears to play a crucial role in establishing these magnetic properties. However, a consistent theoretical picture cannot be drawn at present.

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