Really? 3 weeks in a fermenter?! Never heard of that but interesting
yeah... You'll see it often mentioned on here on HBT.
The idea is to skip secondary altogether and do it all in the primary fermentation vessel. Active fermentation can be done rather quickly (~1 week or so). That's when most of the available sugars are consumed by the yeast. Then the yeast, still hungry, start to work on some of the secondary (less desirable) products of fermentation... "cleaning up after themselves" is the common phrase you'll see.
Then, when those are gone, the yeast drop out of suspension and go dormant... even without a cold crash (albeit slower). It can take a week or more after active primary.... Thus 3 weeks in the fermenter, to be safe.
You can have great beer in less than 3 weeks, especially if you temp control fermentation, cold crash, and force carb in a keg. But letting a beer go longer in primary doesn't hurt and can actually help clean up some potentially off flavors if things weren't 100% ideal in the first few days of fermentation. Especially on bigger beers.
2 weeks to carb in the bottle at 70ish F (some say 3) and 1 week in the fridge to get the C02 fully into solution.
3-2-1
adjust as necessary to fit your process.