Limestone Calcined Clay Cement (LC3) has emerged as a promising low-carbon alternative in the cement and concrete industry. Incorporating expansive agents such as magnesium oxide (MgO) or calcium oxide (CaO) into LC3 formulations may offer additional sustainability benefits by mitigating shrinkage, enhancing crack resistance, improving self-healing capacity, and thereby extending service life. However, as LC3 is known for limited early-age strength development, it is essential to understand how these additives influence early-age hydration. Hence, this study investigates the influence of MgO and CaO on the early-age hydration behaviour of LC3 pastes. Initial and final setting times were determined using an automatic Vicat apparatus to assess stiffness evolution, while isothermal calorimetry was performed for 72 h to evaluate hydration kinetics. Heat flow curves were analysed with a focus on the changes in induction period, the rate of cement hydration, energy release during the main hydration peak, and cumulative heat energy release. The results revealed that MgO significantly delayed both initial and final setting times and postponed the onset of the main hydration peak in LC3 systems. In contrast, CaO demonstrated accelerated and enhanced early hydration activity which explains its early stiffening effect. However, the study strongly emphasizes the importance of optimizing additive selection based on both early and long-term performance criteria for LC3-based cementitious systems.