Acceptable to do significant work on PhD before officially starting?
Your question is based on a false premise.

In many countries (virtually all?) where the nominal PhD duration is 3 years, students are actually paid during their PhD, through a scholarship or a salary, and there is no tuition at all or a minimal one.

It doesn't make any sense, thus, to spend 3 years working independently, with no support from the adviser and for free.

Whether that would be accepted is highly dependent on the country or on the specific university regulations, and we cannot give a general answer. It seems that in my country, Italy, it was once allowed, but now it is no longer possible, by law (I didn't check very carefully though).