I want to know how the assassins from the first three games are related to Desmond. I've tried to find it on the Assassin's Creed Wiki without success.
Altaïr Ibn-La'Ahad (Assassin's Creed) is an ancestor to Desmond Miles through the maternal line.
The Auditore family (Assassin's Creed 2) and the Kenway family (Assassin's Creed 3 and 4) intertwined to become common ancestors to Desmond Miles through the paternal line.
Since you mentioned the wiki, you can find this information in the Desmond Miles article under "Ancestry".