Which should I use? Stepper or DC motor?
3 sweeps per second means that you are going to be spending a large part of the time accelerating the wiper. You need to build this into your torque calculation and it will lead to a requirement for very high starting torque.
A series wound DC motor will probably be most suitable for this.
You have not told us where your requirement has come from, perhaps a rotating glass clear view screen would be more suitable.