Three

Three is a recurring number in the Harry Potter series and may have magical properties like Seven.

Friendship Groups

 * Harry, Ron and Hermione are very close friends and are often reffered to as The Golden Trio
 * Neville Longbottom, Luna Lovegood and Ginny Wealsy stayed behind at Hogwarts during the 1997-1998 school year and reformed Dumbledore's Army
 * Draco Malfoy, Vincent Crabbe and Gregory Goyle are the main reavels of the Golden Trio and are members of families who have Death Eaters links
 * Hannah Abbott, Justin Finch-Fletchley and Ernest Macmillan are the Hufflepuff trio
 * Three members of the Marauders that remained loyal to the Order of the Phoenix - Remus Lupin, James Potter and Sirius Black

Families

 * There are three members of the Dursley family - Petunia Dursley, Vernon Dursley and Dudley Dursley
 * There are three members of the Granger family - Mr Granger, Mrs Granger' and Hermione Granger
 * Arthur Weasley has two brothers
 * There are three members of the Malfoy family - Narcissa Malfoy, Lucius Malfoy and Draco Malfoy
 * There are three members of the Potter Family - James Potter, Lily Potter and Harry Potter
 * There are three members of the Tonks family - Andromeda Tonks, Ted Tonks and Nymphadora Tonks
 * There are three members of the Lupin family - Remus Lupin, Nymphadora Tonks and Teddy Lupin
 * Harry Potter and Ginny Wealsy had three children James Potter, Albus Potter and Lily Potter
 * There are three Black sisters - Andromeda Tonks, Bellatrix Lestrange and Narcissa Malfoy
 * There are three brothers in the Tale of the Three Brothers

Other

 * There are three known headmasters of Hogwarts during Harry's lifetime - Albus Dumbledore, Severus Snape and Minerva McGonagall
 * There are three founders of Hogwarts who disagree with Salazar Slytherin on his views of Blood Purity - Godric Gryffindor, Helga Hufflepuff and Rowena Ravenclaw.
 * There are three Chasers on a Quidditch team
 * There are three House-elves who are important to the story - Dobby, Winky and Kreacher

Magical Objects

 * Ollivander uses three cores - Dragon Heartstring, Unicorn hair and Phoenix feather
 * There are three Unforgivable Curses - Killing Curse, Cruciatus Curse and Imperius Curse
 * There are three Deathly Hallows - Elder Wand, Resurrection Stone and Invisibility Cloak

Other

 * There are three organisations eblished the during the second wizarding war Death Eaters, Dumbledore's Army and The Order of the Phoenix
 * There are three positions Snape held during his time at Hogwarts - Posions, Master, Death Against the Dark Arts teacher and headmaster