What were the three miracles that St. Stephen performed to be canonized? Why was saint Stephen canonized? I know that you have to perform three miracles   to be canonized, but I don't know what his were.
Veneration of martyrs (such as Saint Stephen) as Saints among the Christian faithful long predated the formal process (to include miracles) which later became a standard and evolved into canonization.  Martyrs were prominent among the early saints venerated by Christians.   


  Among other things, persecution sparked the devotion of the saints,
  facilitated the rapid growth and spread of Christianity, prompted
  defenses and explanations of Christianity (the "apologies") and, in
  its aftermath, raised fundamental questions about the nature of the
  church.  


Put a different way, you have asked an anachronistic question.  There was no formal requirement for having performed three miracles when Stephen was recognized as a saint. (credit to @bruisedreed, for how to say that concisely).  

You will find related questions here and also here.