Is it true that the Prophet, may God’s prayers and peace be upon him, was weighed against three prophets, and he outweighed them, and in the end, if he were to be weighed against all the inhabitants of the heavens and the earth, he would outweigh them?

There is no hadith (definition) with this meaning. The closest narration to it is what was reported by Ibn Ishaq that the Prophet, peace and blessings be upon him, said: "I am the supplication of my father Abraham, and the glad tidings of Jesus, peace be upon them both. My mother saw, when she was pregnant with me, that a light emanated from her, illuminating the palaces of Syria. I was nursed among Banu Sa'd ibn Bakr. While I was with our flock, two men came to me, dressed in white garments, carrying a golden basin full of snow. They laid me down, split open my stomach, and then extracted my heart. They split it open and removed a black clot from it and threw it away. Then they washed my heart and stomach with that snow until they purified it, and then restored it as it was. Then one of them said to the other: 'Weigh him against ten of his nation.' So he weighed me against ten, and I outweighed them. Then he said: 'Weigh him against a hundred of his nation.' So he weighed me against a hundred, and I outweighed them. Then he said: 'Weigh him against a thousand of his nation.' So he weighed me against a thousand, and I outweighed them. Then he said: 'Leave him, for if you were to weigh him against his entire nation, he would outweigh them all.'" Al-Albani graded this hadith as authentic.