Explanation:
the circumference of a complete circle (360°) is
2×pi×r
in our case
2×pi×3 = 6pi (= 18.84955592... ft)
5 ft is now what fraction of the whole circumference ?
because the corresponding arc angle is the same fraction of 360°.
5 ft are 5/(6pi) of the whole circumference.
so, the arc angle AB is
5/(6pi) × 360 = 5×60/pi = 300/pi = 95.49296586...°
but maybe you need the angle in radians instead of degrees ?
then remember, 360° = 2pi radians.
and then the arc angle AB is
5/(6pi) × 2pi = 10/6 = 5/3 = 1.666666... radians.