What are these three things Buddha cannot do? According to this page from BuddhaNet, it is mentioned that there are three things Buddha cannot do. They are not mentioned in there and I don't know how reliable the source is. 


  
  The Buddha is omniscient but he is not omnipotent. He is capable of innumerable feats but there are three things he cannot do. Also, a
  Buddha does not claim to be a creator of lives or the Universe.
  


I just found this question discussed on Dhamma Wheel. However, most of the replies are opinion-based.
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.