Question: How many versions of a decree issued at Memphis, Egypt was the key to deciphering Egyptian hieroglyphs for the French historian in the early attempts?

Context paragraphs:
Illustrated Hieroglyphics Handbook: The Illustrated Hieroglyphics Handbook is part of a new genre of books focused on Egyptian hieroglyphs.  The book is a graphics based book with four to seven word examples of each Egyptian hieroglyph; the words are graphically explained for each component of the word, and links to the other entries in the book; each hieroglyph is in "extreme-artistic-detail" and can vary for each hieroglyph, word-to-word.  The determinatives ending a word are explained, (if there is one).  Some determinatives are specific to individual trades, i.e. metallurgy, for example and are not in the Gardiner's sign list of Egyptian hieroglyphs.

Rosetta Stone: The Rosetta Stone is a granodiorite stele, found in 1799, inscribed with three versions of a decree issued at Memphis, Egypt in 196 BC during the Ptolemaic dynasty on behalf of King Ptolemy V.  The top and middle texts are in Ancient Egyptian using hieroglyphic script and Demotic script, respectively, while the bottom is in Ancient Greek.  As the decree has only minor differences between the three versions, the Rosetta Stone proved to be the key to deciphering Egyptian hieroglyphs.

Hieroglyph: A hieroglyph (Greek for "sacred writing") is a character of the ancient Egyptian writing system.  Logographic scripts that are pictographic in form in a way reminiscent of ancient Egyptian are also sometimes called "hieroglyphs".  In Neoplatonism, especially during the Renaissance, a "hieroglyph" was an artistic representation of an esoteric idea, which Neoplatonists believed actual Egyptian hieroglyphs to be.  The word "hieroglyphics" refer to a hieroglyphic script.  Unlike English, the hieroglyphic script was spelt exactly as it was sounded out.  Only those privileged with an extensive education (i.e. the Pharaoh, nobility and priests) were able to read and write hieroglyphs; others used simpler 'joined-up' versions: demotic and hieratic script.

Meroitic alphabet: The Meroitic script refers to two alphasyllabaric scripts developed to write the Kushite language at the beginning of the Meroitic Period (3rd century BCE) of the Kingdom of Kush.  The two scripts are Meroitic Cursive derived of Demotic Egyptian and Meroitic Hieroglyphics derived of Egyptian Hieroglyphs.  Meroitic Cursive is the most widely attested script, comprising ~90% of all inscriptions, and antedates, by a century or more, the earliest, surviving Meroitic hieroglyphic inscription.  Greek historian Diodorus Siculus (about 50 BCE) described the two scripts in his Bibliotheca historica, Book III - "(North) Africa", Chapter 4.  The last known Meroitic inscription is the Meroitic Cursive inscription of the Blemmye (see Beja people) king, Kharamadoye, from a column in the Kalabsha temple (REM 0094), which has recently been re-dated to 410/ 450 of the 5th century CE.  Before the Meroitic Period, Egyptian hieroglyphs were used to write Kushite names and lexical items.

The Hieroglyphs of Ancient Egypt: The Hieroglyphs of Ancient Egypt is part of a new genre of books focussed on Egyptian hieroglyphs.  The book is a primer on hieroglyphs from the viewpoint of topical use within history, with example photographs or illustrations.  The later chapters discuss the hieroglyphs and the history of their translation.

Anatolian hieroglyphs: Anatolian hieroglyphs are an indigenous logographic script native to central Anatolia, consisting of some 500 signs.  They were once commonly known as Hittite hieroglyphs, but the language they encode proved to be Luwian, not Hittite, and the term Luwian hieroglyphs is used in English publications.  They are typologically similar to Egyptian hieroglyphs, but do not derive graphically from that script, and they are not known to have played the sacred role of hieroglyphs in Egypt.  There is no demonstrable connection to Hittite cuneiform.

Rosetta Stone decree: The Decree of Memphis (Ptolemy V) is the third Memphis decree, the first being for his grandfather Ptolemy III (Decree of Canopus), and the second being, Decree of Memphis (Ptolemy IV).  The Decree for Ptolemy V is the text recorded on the Rosetta Stone (issued at Memphis in 196 BC) in the Egyptian hieroglyphs, Demotic (Egyptian) script, and Greek.

Gabriel de La Porte du Theil: Francois-Jean-Gabriel de La Porte du Theil (16 July 1742 in Paris – 28 May 1815) was a French historian.  He played a role in the early attempts to decipher the Rosetta Stone.

Decree of Canopus: The Decree of Canopus is a trilingual inscription in three scripts, which dates from the Ptolemaic period of Ancient Egypt.  It was written in three writing systems: Egyptian hieroglyphs, demotic, and Greek, on several ancient Egyptian memorial stones, or steles.  The inscription is a record of a great assembly of priests held at Canopus, Egypt, in 238 BCE.  Their decree honoured Pharaoh Ptolemy III Euergetes; Queen Berenice, his wife; and Princess Berenice.

Francesco Salvolini: Francesco Salvolini (Faenza 1810February 1838) (also known as François Salvolini) was a scholar of Ancient Egyptian hieroglyphs who worked with Jean-François Champollion on deciphering hieroglyphs near the end of the latter's life.  He is known to have been in possession of some of Champollion's manuscripts and to have used them as a basis for his own subsequent publications on the subject, claiming the work as his own.

Supporting paragraphs: Gabriel de La Porte du Theil Rosetta Stone
Answer: three versions