Jiub's Opus Page 3 is a quest item in Skyrim: Dawnguard. It is needed to complete Impatience of a Saint
Location
To find Jiubs Opus page #3, go west from the opening in the main wall, and through a small opening in a building. Go towards house with purple crystal hovering on the top. Climb two flights of stairs and find the page at the base of a chest on the roof.[1]
Visual Help/Screenshots
This is how the house with the purple crystal above it looks like:
