Vitamin D and zinc are substances that have tolerable upper intake levels. This means that when several supplements containing the same ingredients are taken at the same time, the total dose may turn out to be higher than recommended — even if each individual product stays within the norm.
If you already take a single-ingredient vitamin D3 supplement or a multivitamin complex that includes zinc, add up the amounts from all sources and compare them with current recommendations for your age and state of health. This is easy to do yourself if the packs state the content in mg or µg and the percentage of the daily reference intake.
Vitamin C is less critical in this respect — it is water-soluble and any excess is excreted in the urine, although very high doses can also cause digestive discomfort.
If you take any medicines or have chronic conditions, it is sensible to consult a specialist before adding any new complex — some medicines interact with zinc or affect the absorption of vitamin D.