You don't need to add any yeast at bottling time, unless, 1)you've been bulk aging for 6 months or more in secondary. 2) You have made an extremely high gravity beer and your yeast may have tuckered out.
But at 4 weeks, 8 weeks, yadda yadda yadda, you don't need to add any.
If a beer isn't carbed and conditioned at 3 weeks, it's just not ready yet. The 3 weeks at 70 degrees
, that that we recommend is the minimum
time it takes for average gravity beers to carbonate and condition. Higher grav beers 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.
And even carbonation doesn't mean that they will not still be green and need more time to condition.
Everything you need to know about carbing and conditioning, can be found here Of Patience and Bottle Conditioning.
With emphasis on the word, "patience."