The advice now is to put them back onto dirty land for a couple of days, in order that they pick up worms,
I know that sounds strange but the point is this.
No wormer kills all worms. When you put sheep who have just been wormed onto clean pasture the worms that are in them are resistant worms and so that's what they put onto the pasture. The resistant worms have no competition from any other worms so they have the sheep to themselves. They have a great time, multiply madly and there you go (well after several repeats of this) a resistant worm population.
If you put recently wormed sheep back onto dirty pasture, they will up a selection of worms, most of which will be susceptible to wormers, some of which will be resistant. The susceptible worms will compete for 'sheep gut space and nutrients' with the resistant worms, keeping the resistant worms in their place, as part of the worm population but not the whole of it.
What's the point of worming at all you might ask. Well, you reduce the numbers by killing lots, and that's what sheep who are scouring need - reduced numbers.
As for not worming every 3 weeks - in certain conditions, in lambs, against Nematodirus, that's what's needed. By the time you see symptoms in this instance, you can have dead lambs.
I find the NADIS forecasts very useful in deciding what needs doing.