Don;t do anything, especially add more sugar..unless you want bottle bombs!!!!!!!!!!
There's a simple reason they aren't carbed yet.
You didn't do anything wrong....
They're Just not really ready yet.
The 3 weeks at 70 is only a "suggestion" or rule of thumb for low to average grav beers....below 1.060...But each beer is different and the gravity the longer it takes.
Read my blog
Of Patience and Bottle Conditioning, for more info...
All you need to do is make sure it's above seventy degrees, and walk away for a couple more weeks.
NOTHING ELSE IS NEEDED....all beer will carb up on their own eventually. It Just takes patience.