The tail wheel is un-locked for sharp turns only using wheel brakes and differential power.
The tail wheel is not used to steer as it has no steering or control, just “free wheeling”.
Yes, the tailwheel must be unlocked for any turns, other than directional control on the runway at speed. Thus, it's important that upon landing, you have the airplane aligned with the runway as the tail comes down, or you may find yourself headed off the runway, with little ability to realign the plane to the runway. There are many airplanes which do not have ground steering, and use braking and sometimes differential power to steer, and they all seem to manage quite well. The DC-3's I have flown also have reverse, as they are turbine powered. This does little to help with steering, though makes stopping on the runway easier, as the brakes of a DC-3 are a little sensitive, where reverse is a bit more intuitive, and may also be used differentially during the roll out. My first taildragger training not only did not have tailwheel steering, but did not have a tailwheel at all, it was a Tiger Moth with a wooden skid, and Moth's don't have true differential braking either. Maintain directional control with the rudder, and it works just fine. The DC-3 rudder is large, and aerodynamically very balanced. It is a little alarming in flight attempting large sideslip angles, as the pedal forces tend to lighten a little with very high rudder deflection. But, as any well designed taildragger, the rudder is very effective to a slow speed on the runway, and works just fine - if you use it!