Possibly Celestial or Elven.
The lore of the Giant Elk from the monster manual is as follows:
The majestic giant elk is rare to the point that its appearance is often taken as a foreshadowing of an important event, such as the birth of a king. Legends tell of gods that take the form of giant elk when visiting the Material Plane. Many cultures therefore believe that to hunt these creatures is to invite divine wrath.
While the text of the Giant Eagle is less direct, their intelligence is clearly inspired by the Giant Eagles of Tolkien, who were servants of celestial beings. As such, it's possible that their languages might be based off of Celestial, and written using a variant of the Celestial script.
Similarly, the lore text of the Giant Owl is as follows:
Giant owls often befriend fey and other sylvan creatures and are guardians of their woodland realms.
Additionally, all three of these types of creatures are listed as potential enounters in the Feywild in Domains of Delight, and that same book notes that Awakened Beasts are common in the Feywild. This suggests that their languages may be based off of Sylvan, which uses the Elven script.
Of course, given their anatomy, it's unlikely that any of them are going to be doing much writing themselves, given the lack of hands to write with!