3/8 is right

all possible:
H H H
H H T
H T T
H T H
T H H
T H T
T T T
T T H

If first column is for Keiko, when Keiko got H, Ephraim has two: HT, TH
when Keiko got T (0 head), Ephraim has one TT,

so 3 out of 8