I heard Raspberrypi is an open source project. Where can I get Raspberry pi 3's complete documentation of hardware design and data sheet? I bought a RPI3 and I want to look whether the Micro USB port's data pins are connected or not.
You'll find all official documentation at raspberrypi.org including hardware docs - those pages typically also link to the ARM Processor Reference Manuals - and some bits and pieces in their respective blog.
Note however that the provided information might be somewhat limited. For potentially numerous reasons the RaspberryPi Foundation provides only simplified schematics for the newer models. So, that is that, as they say. It's of course always worth to apply your google-fu to search for additional resources on the web including the various bulletin boards.
I heard Raspberrypi is an open source project.
Now that slightly exaggerates the intents of the Raspberry Pi Foundation. I take it that the key purpose is the promotion of teaching basic computer science in schools or developing countries. Granted many hobbyists use the Pi for numerous applications too. But neither requires a full open source hardware platform (albeit it would be quite nice in many cases).
It is also noteworthy that the Pi requires a binary blob to boot. That's a piece of firmware that's is also not exactly open source. See here for more details: Level of Hackability of raspberry pi