Ω={1,2,3};
ntrials=10000;
iexch=0;
itot=0;
Do[
iprize=RandomInteger[{1,3}];
ichoice=RandomInteger[{1,3}];
a1={iprize};
a2={ichoice};
au=Union[a1,a2];
auc=Complement[Ω,au];
nc=Length[auc];
ilook=RandomChoice[auc,1][[1]];
iwin=If[iprize==ichoice , If[iexch==0,1,0],If[iexch==0,0,1]];
If[iwin==1,itot=itot+1,itot=itot]
(**Print[iprize,"  ", ichoice,"  ",iwin, "  ", itot]**),{i,1,ntrials}]
itot
1.0*itot/ntrials