Kuzan was introduced first, Kuzan was an opponent of Old Garp, Kuzan was decided to be FA through Sengoku's words, Kuzan is the smallest of the C3 and the Youngest, Kuzan was paired with Jozu in the war.
Kizaru was introduced second, Kizaru was an opponent of current Luffy, Kizaru does not care about the FA position as he prefers receving orders than giving orders, Kizaru is the second tallest of the C3, and he was paired with Marco in MF.
Akainu was introduced last, Akainu still has no opponent but it's speculated to be Dragon in the near future, Akainu won the position through strength, Akainu is the tallest out of the C3 and was the opponent of WB.
WB> Marco > Kuzan
Dragon > Current Luffy >= Old Garp (estimative)
Height is a bonus.
Order of appearance is a bonus.