Inno Setup MsgBox with three buttons and three outcomes

You could write multiple if statements, but you'd have to store the returned value into a variable and check that variable value. But as @Sertac mentioned in his comment, you can use a case statement, which better describes the aim in your code, for instance:

case SuppressibleMsgBox('Text', mbConfirmation, MB_YESNOCANCEL, IDYES) of
  IDYES:
  begin
    { user pressed Yes }
  end;
  IDNO:
  begin
    { user pressed No }
  end;
  IDCANCEL:
  begin
    { user pressed Cancel }
  end;
end;

Out of curiosity with multiple if statements it could be:

var
  MsgResult: Integer;
begin
  MsgResult := SuppressibleMsgBox('Text', mbConfirmation, MB_YESNOCANCEL, IDYES);

  if MsgResult = IDYES then
  begin
    { user pressed Yes }
  end
  else
  if MsgResult = IDNO then
  begin
    { user pressed No }
  end
  else
  if MsgResult = IDCANCEL then
  begin
    { user pressed Cancel }
  end;
end;