The group of men – four, according to some Vatican reports, three according to the Vatican's newspaper – joined Francis on Tuesday as he gave his morning Mass and then ate breakfast with him.
The Vatican paper, L'Osservatore Romano, said the men, a Pole, a Slovak and a Czech, were sleeping under the portico outside the Vatican's press center when they were approached by Archbishop Konrad Krajewski, who distributes charitable contributions for the pope.
"Would you like to come to Pope Francis' birthday party?" Krajewski reportedly asked them. "After a moment of bewilderment and wonder, they started packing up their beds, pieces of cardboard and covers which were arranged to protect them from the bitter cold of the Roman night."
The three men, who are in their 40s, one carrying his dog, put their bags into the archbishop's car and climbed in, with the dog in the middle for the ride through the Vatican gates to join Francis and the staff of his residence, who also joined him for Mass.
During breakfast, one of the men told Francis, "It's worth being a tramp because you are received by the pope," according to L'Osservatore Romano.
Francis' devotion to the poor, which he restated last month in an Apostolic Exhortation, was marked in his home city of Buenos Aires on Tuesday by locals who erected a "missionary tent" in an impoverished neighborhood to recall how the then-Archbishop Jorge Mario Bergoglio celebrated Mass there once a year.
In Rome, Francis was preparing to receive a delegation from the soccer team he supports, Argentina's San Lorenzo, which has just won a league championship.
He also appeared to give himself a birthday present, skipping usual Vatican procedures and using a decree to grant sainthood to Father Pierre Favre, a 16th century Jesuit priest whom he admires.
Favre was beatified in 1872. The decree allowed him to make the jump to sainthood without the usual miracle being attributed to him.