answer:
3 units to the left of 0 on a number line
Explanation:
your asking for the opposite of positive 3 which just means your looking for a negative 3 ( also written like this -3 )
and going left on a number line means going down on a number line so if your going into the negatives if you started at 0
here is a small number line to help me explain this better
<----|----|----|----|----|----|----|----|----|----|----|---->
-5 -4 -3 -2 -1 0 1 2 3 4 5
* ( the star is you and your starting off at the #0 )
<|-(-1)-(-1)-(-1)- = -3 ( taking 3 jumps to the left )
*
( and now the star is at -3 which was the number you needed )
i hope this helps you and if you need anything else then just tell me! :)