  The models of large extra compact dimensions, as suggested by Arkani-Hamed,
Dimopoulos and Dvali, predict exciting phenomenological consequences with
gravitational interactions becoming strong at the TeV scale. Such theories can
be tested at the existing and future colliders. In this paper, we study the
contribution of virtual Kaluza-Klein excitations (both spin-0 and 2) in the
process $e^+e^- \to HHZ$ and $ e^+e^- \to HH\gamma $ at future linear collider
(NLC). We find that the virtual exchange KK gravitons can modify the
cross-section $\sigma(e^+e^- \to HHZ)$ significantly from its Standard Model
value and will allow the effective string scale to be probed up to 6.6 TeV. The
second process is absent at the tree level in the standard model, and can
therefore be used to put limits on the effective string scale of 7.4 TeV.
