Its not designed to be used like that. If you are intent on doing this then please make sure you take a multivitamin and mineral to replace the ones you are not getting from slimfast.
I know you want the weight off but in another post you said you feel deprived if you can't have food, deprivation is in my opinion the biggest diet breaker...800 cals will see the weight come off quick but how will you cope without food. I think your original plan of 2 shakes and a meal would be a better option.
I'm not having a pop at you but i'd hate to see you fall off the wagon because you're feeling deprived. Have you tried the 3-2-1 plan and failed? If not give it a go before going so extreme. This is a long tough journey that doesn't need to be made any tougher by being so strict.