The water will be shot by then
The water is already shot...but it's not un-recoverable. Its not going to get much worse in 3 days...
And seriously - stop going to the pool store - all they want to do is sell you stuff. Why do you care about Calcium Hardness in a Vinyl pool ?
How much ammonia was indicated by the test (in PPM). Multiply that by 10 - thats how much FC you need to neutralize it.
If you don't have a test - go to anywhere that sells pet supplies - you should be able to pick up an aquarium Ammonia test for under $10.
So, if its 3PPM Ammonia, you need 30PPM FC to neutralize it. In your 20,000 gallons, thats about 5 gallons of 12.5%
If you have ammonia - do not add anything with stabilizer (CYA) - it'll make things worse.
Add 2 gallons of Chlorine, wait 30 minutes, retest your FC (which is why you need the test kit)...
If it's still at 0FC, add another 2 gallons. Test again. Repeat until you have FC left after 30 minutes - you should be aiming for around 10PPM FC (which again, is why you need the test kit)
At that point, you should have removed all the ammonia, and you'll be at 'SLAM' level for <20PPM CYA
Then follow the SLAM process in the Pool School until your water is perfect again.