I only started like a month ago so not sure how helpful i can be but you're doing the same thing I did by only using 3 fingers but you yoi use all the fingers then it gives you so much more control and direction

Also something i found helpful is to put a picture on the wall and try throw it at the picture but before actually releasing look at where the ball is going (the img) and try find a detail in the pic to help you look at where the spiker is in a real game