  A precise formula for the elliptic genus of three E-strings is presented. The
related refined free energy coincides with the result calculated from
topological string on local half K3 Calabi-Yau threefold up to genus twelve.
The elliptic genus of three heterotic strings computed from M9 domain walls
matches with the result from orbifold formula to high orders. This confirms the
n=3 case of the recent conjecture that n pairs of E-strings can recombine into
n heterotic strings.
