You'll need to clean out much more than 3 GB too afaik. I believe you need double the space to download and install something on Wii U, that was a painful thing I learned with Xenoblade X on my 8 GB system.
If you need 6gb to install this thing then it will be impossible to play on a 8gb system without a hard drive, least amount you can get is like 4gb with all the firmware updates..ffs nintendo