We construct an effective three dimensional theory for the MSSM at high
temperatures in the limit of large-mA. We analyse the two-loop effective
potential of the 3D theory for the case of a light right handed stop to
determine the precise region in the mh-mt~R plane for which
the sphaleron constraint for preservation of the baryon asymmetry is satisfied.
We also compare with results previously obtained usind 3D and 4D calculations
of the effective potential. A two-stage phase transition still persists for a
small range of values of mt~R. The allowed region requires a
value of m_{\tilde{t}_{R}} \lsi m_{t} and m_{h} \lsi 100 (110) GeV for
mQ=300 GeV (1 TeV).Comment: 40 pages, 6 Postcsript figures, uses eps