First off, you shouldn't have gone 3 weeks without adding chlorine, but you know that now!
Try this..in the evening time, throw in 1lb powered shock, 4 oz of algaecide & 4 oz of clarifier, then let the filter run all night.
Per 10,000 gallons.
In the morning, you should see stuff on the pool floor.
Vacuum to waste.
Then continue to add chlorine daily, run filter, at least for 8 hours a day and vacuum when needed.
Also, backwash you filter when reading get 5-7 psi over original reading.
Hope this helps!