William Hartnell was the first Doctor, this much you may know. Let’s see your trivia knowledge a bit more, did you know that William Hartnell reprised the role in The Three Doctors? It was written for him due to failing health, to not appear extensively.
Did you know that Richard Hurdnall played the role of the Doctor in The Five Doctors?
The first Doctor, although younger, appeared to be more wise than the rest of the Doctors that were his successors. The role of the first Doctor to be played by someone other than the original Hartnell was approved by his widow, Heather.
Originally the plot for the reunion special did not include too much of the first Doctor’s role, but as Tom Baker declined the role, his part was expanded.
So in reality, The Five Doctors, were really four, archival footage was used for Tom Baker’s fourth incarnation of the role. And if you really want to be technical, since Richard Hurdnall was not the original actor, it’s just three doctors all over again.