What is the history behind calling the position wing three-quarters in rugby union?

Answer 1

Because they had wing forward now called flankers and then had to distinguish the backs by calling them wing three quarters as the were in the three quarter position i..e Scrum half and Outside haf are called "Half Backs" and the full back is "full back" the centres were classed as three quarter back.

Answer 2

the 1st and 2nd five eighths come between the scrum half and the

wing three quarters. So 5/8 comes between 1/2 and 3/4