From the CoE 3.26 executable:

Download[www.dropbox.com] the Conquest of Elysium 3 complete definitions of all the monsters/units in the standard game.

For the curious among you: the corresponding CoE4 data in the CoE4 forum.
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Like before, the format is c3m files (plain text files) with pseudo-mod code. And, for the best reading experience, I recommend a text editor with CoE syntax highlighting, such as the schemes I have made for for Gedit and Notepad++ – or, if nothing else, the syntax highlighting of just a Unix shell script. (The last one will at least make comments and string literals stand out from the rest.)

Whatever you do, I would advise you don't use the Notepad that comes with Windows. (If you're on Windows, you'll be much better off with Notepad++[notepad-plus-plus.org].) Microsoft Notepad and CoE modding (or just about any serious text processing) are two things that simply don't belong in the same sentence.
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Other data:

• Terrain data.
  
• Class and recruitment data.
  
• Weapon, spell and item data.
  
  
• Also, a spreadsheet with the data for terrain, items, weapons and spells in a more player-friendly format, plus, for modders, lists of sounds, fxIDs and monster flags.