SeedRandom[1234];
ClearAll[ComputerChoose, HumanChoose]
ComputerChoose[n_] := If[n < 18, RandomChoice[{1, 2, 3}], 21 - n]
HumanChoose[] := ChoiceDialog["How many?", {1 -> 1, 2 -> 2, 3 -> 3, "Quit" -> -1}]
runningtotal = 0;
whofirst = ChoiceDialog["Who goes first?", {"You" -> 1, "Computer" -> 2}];
While[runningtotal < 21,
 If[whofirst == 1,
   choice = HumanChoose[];
   If[choice == -1, Break[]];
   Print["You choose = ", choice];
   runningtotal += choice;
   Print["Running total = ", runningtotal];
   If[runningtotal == 21, Print["You won!"]; Break[]];
   choice = ComputerChoose[runningtotal];
   Print["Computer choose = ", choice];
   runningtotal += choice;
   Print["Running total = ", runningtotal];
   If[runningtotal == 21, Print["Computer won!"]; Break[]];
   ,
   choice = ComputerChoose[runningtotal];
   Print["Computer choose = ", choice];
   runningtotal += choice;
   Print["Running total = ", runningtotal];
   If[runningtotal == 21, Print["Computer won!"]; Break[]];
   choice = HumanChoose[];
   If[choice == -1, Break[]];
   Print["You choose = ", choice];
   runningtotal += choice;
   Print["Running total = ", runningtotal];
   If[runningtotal == 21, Print["You won!"]; Break[]];
   ];
 ]