A flange inside of a 3" pipe would restrict the size of the drain too much, and I don't think there is one anyway. A toilet on a platform is the mark of a hack job and I would advise avoiding that. Chip concrete around the pipe to expose enough to accommodate a 3" flange, clean it off so the flange will fit over it. You probably will want to rent a rotary hammer/drill to do this and work carefully so as not to break the pipe. It will take some time and effort, but it will be the best way.