We previously thought that is was impossible to install in the previous models of Time Capsule a disk of more than 2 TB. Some readers tried and were unable to format the disk, which appeared to have a capacity of only 750 GB.
Jef was however successful:
I personally installed a 3 TB WD Caviar Green 3To (by chance!), when it came out 5 months ago, in a 1 TB Time Capsule that I bought around that time as well. I formatted it with Airport Utility to have the 3 partitions that are necessary for Time Machine and it was immediately recognized as a 3 TB disk. I've been using it ever since without any problem.
It appears that the older models of Time Capsule can support disks of that capacity but are sometimes unable to format them. The disks inside the Time Capsule have 3 partitions. You must replicate them on the new disk, otherwise it will not work properly.