CO$_2$ insertion into tri(mu-hydrido) triiron(II) clusters ligated by a tris($\beta$-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO$_2$ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe$_3$H$_3$L$^{OMe/Me}$, 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe$_3$H$_3$L$^{Et/Me}$(2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO$_2$ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD$_3$ requires Fe-O bond formation in the product to offset an endergonic CO$_2$ insertion. $\mu_3$-hydride coordination is noted to lower the activation barrier for the first CO$_2$ insertion event in DFT calculations