Overcoming Difficulties with the 3'
3' (3 draw half step bend) is an important note for blues harmonica players, and can be difficult to control for new players to bending. The common error is in thinking that the 3 bend is a deep bend and requires more use of the back of the tongue, in the back of the mouth. This is accurate for the deeper 3 draw bends (3" and 3'''), but not for the 3'.
Work your 6 bend for a while... starting unbent and then slowly bending down to the deepest point. You'll notice that you're using more of the front of the tongue, further forward in the mouth. In fact, if you're not, the 6 won't bend at all, or will bend quickly and then pop back to its natural vibrating pitch (you've gone past that reed's ability to bend).
Work that 6 bend range for a while, until you can visualize what your tongue is doing and where it is located in your mouth.
Now use the same motion for the 3' and you'll find you'll have much more control over the bend range from 3 to 3'. You may need to move or hum your tongue slightly further back in the mouth to achieve this bend, but only slightly.
Once you've mastered the 3' you can focus on then using more of the middle of the tongue in the middle of the mouth for the 3" and then the back of the tongue further back in your mouth for the 3'''. The challenge is of course sliding smoothly for this entire bend range.