Why was Bapusaheb Dev so upset that three uninvited beggars crashed his religious feast?

This premise is incorrect. Bapusaheb Dev was not upset because uninvited guests attended his feast; in fact, he personally invited them. Chapter 40 recounts that when a Sanyasi appeared, Dev told him, 'come with the children around twelve o'clock and have your meal.' Dev's subsequent disappointment and letter to Jog had nothing to do with the guests' presence, but rather with his own belief that Sai Baba had failed to keep His promise to come. He did not realize that the Sanyasi he had served was Sai Baba himself in disguise.