  Understanding the mechanisms of efficient and robust energy transfer in
organic systems provides us with new insights for the optimal design of
artificial systems. In this paper, we explore the dynamics of excitation energy
transfer (EET) through a complex quantum network by a toy model consisting of
three sites coupled to environments. We study how the coherent evolution and
the noise-induced decoherence work together to reach efficient EET and
illustrate the role of the phase factor attached to the coupling constant in
the EET. By comparing the differences between the Markovian and non-Markovian
dynamics, we discuss the effect of environment and the spatial structure of
system on the dynamics and the efficiency of EET. A intuitive picture is given
to show how the exciton is transferred through the system. Employing the simple
model, we show the robustness of EET efficiency under the influence of the
environment and elucidate the important role of quantum coherence in EET. We go
further to study the quantum feature of the EET dynamics by {\it quantumness}
and show the importance of quantum coherence from a new respect. We calculate
the energy current in the EET and its quantumness, results for different system
parameters are presented and discussed.
