The MBR structure that Microsoft used to use for discs has a maximum of 2TB… there is no room for any entries in the MBR past 2TB.
Drives that exceed 2TB use a different structure (GPT). Basically GPT is a “trick” that hard drive manufacturers have come up with to get past the 2TB limitation… but it’s not really “built in” to operating systems yet.
When you tried to “play around” with the volume sizes, Windows turned the drive back into MBR instead of GPT. Since the MBR structure will only let you allocate a total of 2TB, regardless of sector size, that’s why the remaining space was split into 250GB that you can use and 750GB that you can’t.
Before: 2.8TB GPT
After Shrinking: 1.8TB MBR + 1TB unallocated space
Because the MBR can only reference a total of 2TB, you can only add another 250GB of that unallocated space to the MBR entries. The other 750GB became unusable.
You’ll have to forget about trying to play with the volumes using Windows, and instead use tools that will allow formatting/partitioning using GPT – then you can use the whole 2.8TB again.