To stop most rifles, you would have to step up to level 4 or some of the exotic stuff like dragon skin.
That is not true. Level IIIa is soft armor and is handgun rated. Level III starts into HARD armor and is Rifle rated. III+ stops most rifle threats. Level 4 adds 30-06 AP, prob not needed for most applications.
I run III+ plates, those stop 7.62x39 and all the 223/556 threats AND 308 M80 ball.