Deus Ex 3 was announced well before Thief 4, but we still don't know much of anything about the Eidos Montreal project. That could change though, with the game possibly gaining a new name.
Siliconera is reporting that Square Enix has trademarked the name Deus Ex: Human Revolution in Europe. While it could potentially be a brand new game in the series, it's more likely to be the official title for the above-mentioned prequel.

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