For the doubting mind or historically curious it is easy to find in Google Search and encyclopedias sources that substantiate claims of forgery for the following three instances. Today they are accepted by virtually all serious historians as falsifications.
THE DONATION OF CONSTANTINE
The Donation of Constantine (Latin: Donatio Constantini) is a forged Roman imperial decree by which the emperor Constantine the Great supposedly transferred authority over Rome and the western part of the Roman Empire to the Pope. Composed probably in the 8th century, it was used, especially in the 13th century, in support of claims of political authority by the papacy.”
The Catholic Encyclopedia affirms: “This document is without doubt a forgery, fabricated somewhere between the years 750 and 850. As early as the fifteenth century its falsity was known and demonstrated.”
“During the dispute between Pope St. Symmachus and the anti-pope Laurentius,” the Catholic Encyclopedia reports, “the adherents of Symmachus drew up four apocryphal writings called the ‘Symmachian Forgeries’. … The object of these forgeries was to produce alleged instances from earlier times to support the whole procedure of the adherents of Symmachus, and, in particular, the position that the Roman bishop could not be judged by any court composed of other bishops.”
“The Pseudo-Isidorean Decretals (or False Decretals) are a set of extensive and influential medieval forgeries, written by a scholar or group of scholars known as Pseudo-Isidore. The authors, who worked under the pseudonym Isidore Mercator, were probably a group of Frankish clerics writing in the second quarter of the ninth century. They aimed to defend the position of bishops against metropolitans and secular authorities by creating false documents purportedly authored by early popes, together with interpolated conciliar documents.”
“False Decretals is a name given to certain apocryphal papal letters contained in a collection of canon laws composed about the middle of the ninth century by an author who uses the pseudonym of Isidore Mercator, in the opening preface to the collection.”(Home Catholic Encyclopedia “New Advent.”)