Why was the third floor forbidden in Harry Potter and Stone?

The third floor in Harry Potter and the Sorcerer's Stone was forbidden because it housed the Philosopher's Stone, the magical artifact that could turn any metal into gold and create the Elixir of Life.

The Stone was being guarded fiercely by Professor Dumbledore and was considered a dangerous and valuable asset. The third floor was therefore designated as a restricted area to keep the Stone safe from unwanted access, especially from Lord Voldemort and his followers who were attempting to steal it.