There are two temp areas that AS will write files to.
1. Is the Temp directory, this by default is the same as your Data folder.
You can check by right clicking the servername in Analysis Manager and then
choose properties. The first tab will show you data folder and temp
2. I also believe some temp files are written to the Bin folder. So make
sure you have enough disk space there as well.
If the data folder for this cube is only 3GB you should have plenty of Disk
space. Make sure your swap file isn't set too large.
How many aggregations are being created at 5%? You may still need to
decrease this if the number of aggs are to large. I can't imagine that you
would have that many aggs at 5% to consume that much disk space.
How much ram is AS taking when you first start AS? What is the process
buffer and read ahead buffer size in the properties of the server?