if 2 equal distance are covered with speeds a, and b
Then the avg speed is harmonic mean of a and b.
so avg speed = 2/(1/a + 1/b)

This can be extended to any number of distances
So if you have a person who is going with speeds a,b,c,d…. over equal distances, d,d,d,… upto n

then the avg speed will be :

n/ ( 1/a + 1/b + 1/c + 1/d + ……………)

and so on..

This can be used to find avg speed over 3 equal distance. if speeds are a,b,c, for 3 equal distances, then avg speed will be:
H.M. of a,b,c, ( HM means harmonic mean)

= 3/ (1/a + 1/b + 1/c )