If you do a 3toe/3toe combination, everything that I've written above goes away. You have to perform a 3toe right off the landing of a triple toe. Unless your first 3toe was perfect you won't have ideal balance or landing speed, you'll be fatigued because of just having landed a jump and even if that landing was perfect you have to perform a triple jump with no preparation.
Also, you can tack a 2toe at the end of the 3lutz even if it wasn't landed well. You cannot tack a 3toe at the end of a 3toe if it wasn't landed well. Unless you're Stefan Lindemann.
If you look at the jump progression of skaters as they go from junior to senior, in the vast majority of cases they first perform 3lutz/2toe before being able to perform any 3/3 combos. Hanyu had a consistent 3axel before he was able to land any 3/3 combos.