The Turbo3 unit is provided for backward compatibility to Turbo Pascal 3.0 only. Every routine in the Turbo3 unit is duplicated or improved upon in other standard units.
By using Turbo3, you gain more compatibility with Turbo Pascal 3.0, but you lose direct access to important new features built into some of the standard routines.
You can still call these standard routines by using the unit override syntax. For example, Turbo3's MemAvail calls the System.MemAvail function even if you are using the Turbo3 unit in your program.