You could just give that user its own file system. The only way to do what you are asking is to set up a quota (and the file system that has the quotas must be its own file system).

Essentially, if you have a 4GB /home you need to give your other users a quota of however much you wish where the total of their quota does not equal 1GB, and then give the other user either no quota or a 3GB quota (giving them a quota can cripple them if they run out of inodes).

Otherwise, create a new file system for user "c" and make it >3GB or set it so there is no reserved space for root. then, make the ownership and permissions for that user conducive to what you want, and change their home directory to that directory OR simply let them use it for application/scratch/whatever space