[lmikle]

Код:

uses Math;

function Min3(x, y, z : Extended) : Extended;
begin
  Result := Min(Min(x,y),z);
end;

var
  x, y, z : Extended;
begin
  x := ...
  y := ...
  z := ...
  ShowMessage(FloatToStr(Min3(x+y, y+z, x+z)));
end;