Can I replace the HDD in a Fusion Drive? I'm thinking about buying a refurbished iMac that has a 1 Tb fusion drive. Is it possible to replace the stock HDD with a 3 Tb drive?
The fusion part is not contained in the drive. There is a separate connector for the SSD on the logic board. You can definitely replace the hard drive with a larger one once you buy it.

Fusion Drive is a marketing term for what's known under the hood as CoreStorage, the technology which, among other things, can combine two drives into one. Apple doesn't pull any hardware tricks to implement this. It's 100% software-based. They have a connector (albeit somewhat proprietary) for the SSD, and another connector for the HDD. Combined with CoreStorage, they form the Fusion Drive.