How does one promote a third queen in an over the board game? Most chess sets come with only one spare queen, and according to the video in the question Is holding the opponent&#39;s captured pieces legal or allowed?, one cannot use an upside down rook, and doing so it results in promotion to a rook.
If two queens already on the board, how can one promote a third one?
Under USCF rule 8F7:, it is acceptable to place a rook upside-down if a spare queen is not available.

If the desired piece is not available to replace a promoted pawn, the player may stop both clocks in order to locate that piece and place it on the board. A player who cannot quickly find such a piece may request the assistance of the director. It is common practice, however, to play using an upside-down rook for a second queen. In the absence of the player’s announcement to the contrary, an upside-down rook shall be considered a queen.

Under FIDE rules, you can't use an upside-down rook as a spare queen, but you can find the answer hidden in the section about the clocks:

6.11.2  A player may stop the chessclock only in order to seek the arbiter’s assistance, for example when promotion has taken place and the piece required is not available.