N=3 Superparticle Model
  We consider the formulation and quantization of the N=3 superparticle model, both with and without central charge. Without the central charge the action possesses U(3) invariance and therefore is naturally quantized in the N=3 harmonic superspace. The quantization reproduces the N=3 supergauge strength multiplets, described by analytic N=3 superfields and a gravitino multiplet as a constrained N=3 chiral superfield. When the central charge is present, it breaks the U(3) R-symmetry of N=3 superalgebra down to SU(2)xU(1), and the corresponding superparticle model is formulated in the N=2 harmonic superspace extended by a pair of extra Grassmann variables. The quantization of such a model leads to the massive BPS N=3 vector multiplet. It is shown that upon additional superfield constraints such multiplet reduces to the massive N=2 vector multiplet. 
I.L. Buchbinder, I.B. Samsonov