When you run the Classic Client on SQL Server, this is a 2 tier installation, not a 3 tier installation. The SQL Server stores the database, and the client sessions connect directly to SQL Server. You could make a case that a terminal server implementation is a 3 tier, but from a NAV perspective the client session still connects to SQL Server directly, so still 2 tiers.
The Application Server is a separate type of NAV client, basically a regular Classic Client without a user interface. Although this is often used to interface NAV with other systems, from a NAV perspective… still 2 tiers.
Where 3 tiers come into play is the RTC implementation. The RTC client connects to the service tier, which in turn connects to SQL Server. THAT’s a 3 tier NAV implementation. You might have a service tier installed while using the Classic Client, but since the classic client does not connect to the service tier, still a 2 tier implementation.
You can use the Classic Client and the RTC at the same time, in fact all NAV development (including RTC development) is done in the Classic Client.
You should probably get your partner involved to help you get set up.