Backing up multiple hard drives to a single partitioned hard drive

Running Win 10 Home, 64 bit. I have three 1 TB hard drives, which are regularly backed up to three USB 1 TB drives. I also have a 4 TB drive and for offsite storage, I’d like to logically divide that up into three drives and be able to copy my three 1 TB drives onto that 4 TB drive. I can partition the 4 TB drive into three unique drive with drive letters, but my backup software (Casper) isn’t able to deal with having the destination drive set up like that. How can I get this done, or, maybe it just isn’t possible?