DON'T DO ANYTHING!!!! Don't add carb tabs unless you want bottle bombs.
Just because your beer is not carbed at 3 weeks, doesn't mean your sugar didn't mix. Despite what most new brewers may think, it is nealry impossible for your priming sugar solution NOT to mix with the 5 gallons of beer. It really doesn't take much motion, just the natural swirlong of the beer as it rises to integrate the two fluids. The people who assume that is the cause of their beer not carbing, are usually just not waiting long enough, and looking for something to blame, and are wrong.
The 3 weeks at 70 degrees
, that we recommend is the minimum
time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer. Lower temperatures take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled, it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here Of Patience and Bottle Conditioning.
With emphasis on the word, "patience."
Read the above blog, and come back to the beer in a couple more weeks.
If a beer isn't carbed by "x number of weeks" you just have to give them ore time. If you added your sugar, then the beer will carb up eventually, it's really a foolroof process. All beers will carb up eventually. A lot of new brewers think they have to "troubleshoot" a bottling issue, when there really is none, the beer knows how to carb itself. In fact if you run beersmiths carbing calculator, some lower grav beers don't even require additional sugar to reach their minimum level of carbonation. Just time.
You don't need to add more sugar, just more time.....