T3i to full frame is quite a big jump, and even if 2,800 USD is quite a big amount of money, it is maybe not that much to make the jump in one step...
My first questions would be
1/What type of pics do you make / want to make?
2/Why do you want a FF?
3/What other accessories do you have?
Usually people making these jumps only have the camera + a few lenses. I would recommend to invest in a descent tripod, trigger, a flash (at least to try if you like flash or not
) and a polarising filter total (800-1200 USD)... It will help you a lot and will be new toys to play with
Regarding the proper upgrade, why not stay on APSC by buying a 70D or a used 60D and invest in lenses that you will be able to keep on your full frame such as a 24, 35 or 85L... (you could maybe have the budget for 2L and a 60D if you already own the tripod and the flash)
Like that next time you whant to change of body, you will be able to really focus of the camera body...
Like that next time you make the jump