hess's law

If you want to find out the standard enthalpy change of formation of MgCO3 directly, you need to use one mole of Mg, C and 1.5 mole of O2 gas. Then, let them to react at high temperature. It is unlikely that you can make one mole of MgCO3 due to the fact that you can only produce MgO and CO2.
So, you cannot control the extent of the reaction as the products cannot be controlled in this case.