What are these three things Buddha cannot do?
The closest I can find so far, from DN 29, are 9 things that enlightened persons (arahants) cannot do, which apply to the Buddha too:


  A mendicant who is perfected—with defilements ended, who has completed
  the spiritual journey, done what had to be done, laid down the burden,
  achieved their own goal, utterly ended the fetters of rebirth, and is
  rightly freed through enlightenment—can’t transgress in nine respects.
  A mendicant with defilements ended can’t deliberately take the life of
  a living creature, take something with the intention to steal, have
  sex, tell a deliberate lie, or store up goods for their own enjoyment
  like they did as a lay person. And they can’t make decisions
  prejudiced by favoritism, hostility, stupidity, or cowardice. A
  mendicant who is perfected can’t transgress in these nine respects.’


The above is the Sujato translation. You can find the Thanissaro translation here.