Vitamin A, B & C: can they mix?
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A well-rounded routine for healthy skin should cover the ABCs: retinol (vitamin A), niacinamide (vitamin B3) and ascorbic acid (vitamin C).
But just because they follow each other in the alphabet, does not mean they should follow each other in application.
Retinol is notoriously unstable in air and light, so should always be delivered in stabilised form to maintain efficacy. If mixed with acidic substances, such as AHAs, BHAs and ascorbic acid (vitamin C), the acidic environment neutralises the retinol, making it less active and therefore, less effective. What a waste!
In addition, mixing these acids with vitamin B3 also inactivates the niacinamide component. It can also cause a chemical reaction that produces "nicotinic acid", and while this is not harmful, it can cause facial flushing. No thanks!
However, retinol + niacinamide? Together, these two act as a powerhouse pairing with complementary benefits, particularly for overnight skin regeneration.
In short: keep A & B together for the night shift, then rise and shine with your vitamin C & other acids in the morning. 
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