Попробуйте такое:


uses graphABC;
var
  x, y: array [1 .. 100] of integer;
  mI, mJ, mK, maxR, R, i, j, k: integer;
begin
  randomize;
  for i := 1 to 100 do
  begin
    x[i] := random(600) + 10;
    y[i] := random(360) + 10;
    circle(x[i], y[i], 2);
  end;
  maxR := 0;
  for i := 1 to 100 do
    for j := 2 to 99 do
      for k := 3 to 98 do
      begin 
        R := abs((x[i]-x[k])*(y[j]-y[k])-(x[j]-x[k])*(y[i]-y[k])div 2);
        if R > maxR then
        begin
          maxR := R;
          if R > mI then mI := i;
          if R > mJ then mJ := j;
          if R > mK then mK := k;
        end;
      end;
  setpencolor(clred);
  line(x[mI], y[mI], x[mJ], y[mJ]);
  line(x[mJ], y[mJ], x[mK], y[mK]);
  line(x[mK], y[mK], x[mI], y[mI]);
end.