What is the best way to begin explaining the adoption story for a 3-year old?

Our son is three years old and we have an open adoption. We currently see his birth mother and siblings about every 8-12 months. Until now he just knows he has other family members in a different City. What is the best way to begin explaining the adoption story for this age? Do we call his siblings brother and sister, do provide all the facts? Appreciate the feedback!