  A formalism for spin observables of the reaction
  $pd\to ~^3He\eta$ is derived in a model independent way.
  The general case with a full set of six independent spin amplitudes is
studied. Furthermore, approximations by five and four spin amplitudes are
investigated in the near threshold region. This region is of great interest to
search for a quasi-bound $^3He-\eta$ state, in particular, by measurement of
energy dependence of relative phases of s- and p-wave amplitudes. Complete
polarization experiments, allowing determination of spin amplitudes, are
analyzed. It is shown that measurement of only analyzing powers and spin
correlation coefficients hardly allows one to separate the s- and p-wave
amplitudes, but additional measurement of polarization transfer coefficients
simplifies this problem. Specific observables, given by products of one s- and
one p-wave amplitudes, are found. Measurement of these observables will provide
new independent information on the $^3He-\eta$ pole position.
