Hey Paul - thanks for your kind words man. Hmmmm…
Bending is such a trip and I know can be so frustrating some times!
Before going further I just wanna take a movement to acknowledge that the -3’‘’ bend is probably the LEAST IMPORTANT of all the draw bends in terms of how often it comes into use. It’s really not necessary in 2nd position or 1st position playing. It is the “tritone” (#4/b5) interval in 3rd position, but the -1’, -2’ and -2", -3’ -3", -4’ and -6’ are all WAAAAY MORE USEFUL COMMONLY PLAYED than the -3’‘’.
Still it’s a worth pursuit to get it, I just want to put it in proper perspective. Kinda like some beginner and intermediate players get so obsessed with overblows when 99% of the harmonica songs and solos most of us love don’t have an overflow in them.
OK, I’ll get off my soapbox now and back to the matter at hand. 
I just happen to have an a harmonica sitting here next to me and experimented with the -3’ -3’’ and -3’‘’ and tried to pay attention to what’s going on in my mouth…
It sounds like you have the right idea -making the back of your throat as big as possible. The one thing I noticed when I was just trying this was paying attention to the tip of my tongue:
-3 tip of tongue near top of bottom teeth
-3’ tip of tongue near bottom of bottom teeth
-3’’ tip of tongue at gum BELOW bottom of bottom teeth
-3’‘’ tip of tongue pointed DOWN into soft tissue at bottom of mouth
Another tip of I heard that could be useful: try whistling the the 4 tones (B Bb A Ab) and notice what’s going on inside your mouth. That could potentially give you a clue…
Above all BE PATIENT.
DON’T LOSE FAITH!!
(It’s so tempting to start believing we’ll NEVER get it!!!)
You WILL GET IT. Just PLAY with it like it’s a game. And keep coming back to it and giving it a whirl.
That’s my advice. Hope it helps! 