My dash is indicating my car is due for a Service A3 maintenance, what is that?

Hello, first - your owners guide has all the details of needed maintenance. It would be a really good idea to become familiar with that, as it will discuss all maintenance -- which you can become aware of and be ready for. The "A" service is a comprehensive "minor" service of inspections and maintenance due, including replacing the engine oil filter and oil. I am not seeing "3" in the manual, so perhaps a local Mercedes specialist can be of assistance, if it is not listed in your owners guide: https://repairpal.com/repair-shops/mercedes-benz-repair-and-service-in-greenville-south-carolina