Option 1 - Another 7970, most powerful option of the 3.
Option 2 - 290x - Current tech and generally around 30% more powerful in games than a single 7970, about 30% slower than 7970 crossfire, not much cost difference than 7970 crossfire after the sale of your current one.
Option 3 - 780ti is a little slower, maybe on par with the 290x so is the extra £100 getting you? (I based this on the MSI Gaming editions), also the most costly option.
My recommendation would be single 290x as you'll be getting 30% more performance in gaming than you already have, better support for larger screens with the 4GB VRAM ^_^