What is the average velocity between 12.00 and 15.00 seconds

The average between 12.00 and 15.00 seconds
can be defined in a few different ways.

Way #1:  (speed)

     Average speed =
         (1/2) (the speed at 12 seconds  +  the speed at 15 seconds) .

Way #2:  (speed)

   Average speed =
       (1/3) (the distance covered between 12.00 and 15.00 seconds)

Way #3:  (velocity)

   Average velocity =
     (1/3) (distance between location at 12 sec and location at 15 sec)
       in the direction from location at 12 sec to location at 15 sec.

That's the best we can do with the combination of given and
not given information in the question.