  We incorporate the effects of mixing arising from the trilinear terms in the
MSSM potential to the effective three dimensional theory for the MSSM at high
temperature in the limit of large $m_{A}$. There are relevant one-loop effects
that modify the 3D parameters of the effective theory.
  We calculate the two-loop effective potential of the 3D theory for the Higgs
and the right handed stop to analyse the possible phase transitions and to
determine the precise region in the $m_{h}$-$m_{\tilde{t}_{2}}$ plane for which
the sphaleron constraint for preservation of the baryon asymmetry is satisfied.
There is an upper bound on the value of the mixing parameter coming from stop
searches. We also compare with previous results obtained using 4D calculations
of the effective potential for the regime of large $m_{Q}$. A two-stage phase
transition persists for a small range of values of $m_{\tilde{t}_{2}}$ for
given values of the mixing parameter and $\tan\beta$. This can further
constrain the allowed region of parameter space. Electroweak baryogenesis
requires a value of $m_{\tilde{t}_{2}}\lsi 170$ and $m_{h} \lsi 105$ GeV for
$m_{Q}=300$ GeV.
