I just wanted to post this because I saw confusion in this topic in several other posts with no definitive answers. Also, some Dell chat tech support reps give incorrect info on this. I was told by a tech support rep (I won't mention his name but definitely a name from India) that the Dell XPS 8300 can only use 2 hard drives. There were a few others I chat with who said the same thing. That is absolutely 100% incorrect. One can already make the speculation that it can support more than 2 hard drives just by counting the number of SATA ports in the motherboard - there are 4. However, I am speaking from my own personal experience in having successfully connected 3 hard drives, not just guessing or speculation. You could probably also connect 4 hard drives if you wanted, but you'd probably want 1 of the 4 SATA ports to be for your CD/DVD-ROM drive.
In another post someone is concerned with where a 3rd hard drive can be screwed into. A 3rd hard drive fits PERFECTLY into the slot just above the 2 hard drive slots (the flexbay slot). I mean perfectly – it is a very snug fit, it's almost as though this slot was designed to fit a hard drive (even though the Dell tech support rep said that this slot could not be used for a hard drive - how wrong he was). The hard drive does not move around (it takes a firm amount of pressure to move it) so there is no need to screw it in - it just stays in place firmly. And it works, I tested it and now I have 3 hard drives on my XPS 8300.
Here is a pic of all 3 hard drives installed. You can see the green underside of the first 2 hard drives and the 3rd hard drive is immediately to the left in this photo (if the computer is upright then it would be the slot above the hard drive slots):
Here is a front view photo:
Notice how the slot fits the hard drive perfectly. Again, it does not move easily without firm pressure so there is no need to screw it in. Note, I could have also put the hard drive into the 2nd CD/DVD-ROM bay but it wouldn't be an exact fit (you'd probably have to buy and put in brackets) and also, why waste this slot in case you do want a 2nd CD/DVD-ROM drive later? The tech support rep said that the flexbay slot is a "dummy" slot and does not have a purpose (but again he said that a hard drive could not go in there, so even this could be wrong info). Anyway I hope this helps someone who is also considering 3 hard drives in their Dell XPS 8300.
My previous computer was a Dell XPS 420 and the chassis design is rather different. The XPS 420 is bigger and there was more room to maneuver and the 2 hard drive slots were very conveniently located - they basically snapped into place. It looks like a 3rd hard drive would have been possible but I never actually tried it with that computer. Being used to the XPS 420 and then going to the XPS 8300, it feels really tight in there. If you have big hands you may have to unscrew and remove the video card when plugging or unplugging the SATA cables into the motherboard (but hopefully you won't have to do this frequently, perhaps only once). I liked the XPS 420 design better (I started other threads about some issues I have with the XPS 8300 chassis design).
I should also mention, the first 2 SATA ports (labelled SATA 0 and SATA 1) are the higher speed ports, SATA 3.0, which means you can connect the faster 6 GB/s hard drives into them. The remaining two ports (labelled SATA 2 and SATA 3) are a slower speed, SATA 2.0, so it doesn't matter if you put a 6 GB/s drive or a 3 GB/s drive into them - it'll still only operate at 3 GB/s. So if you're on a budget then it's probably more economical to get a 3 GB/s drive for your 3rd drive.