Your problem with 3 draw is a common one. The draw reed is the longest and it needs more control than the other reeds. The only tipp I can give: Keep on practicing. It did this literally for week till I started to get the half step bend right.
Sometimes it helps to grab a lower tuned harp (e.g. A major) which is easier to bend the the C major harp you are practicing with.
Have fun – it’s a journey