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Of course the three pieces belong to one side :) and the extra piece had been obtained by underpromotion
I read somewhere that in the 19th century the chess master Mackenzie played a masterpiece where he underpromoted about 4 times. I don't know the opponent or the game. It might hold the underpromotion record if I recall from a Chernev book.
There are problems and studies where three Bishops or Knights appear on the board, and there are real games where a third piece was obtained by underpromotion, but I don't know of any games where there were three of the same minor on the board at the same time. I don't believe I've ever seen even a puzzle with three Rooks in the solution.