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.