3 weeks is doable, people are treating it like a long holiday.
3 months- I think people will give up by that point and more and more will flout the rules. People just won’t stay away from their family for that long. They’ll go out, socialise more.
The virus will spread quicker, everyone who is going to catch it, will catch it and then life will get back to normal. Unfortunately more people will die that way but I also think people are selfish enough to say ‘it’s gonna happen anyway, and this way I don’t have to self isolate for another 3 months’