If you can run the equipment, I would do all of the following:

1. Begin circulating the pool.

2. Using a cheap OTO/phenol red kit, make sure the pH is between 7 and 8. Add bleach till you reach a dark yellow result on the kit.

3. If you can, uncover a portion of the pool near a pool return or eyeball. If possible, adjust the eyeball to 'riffle' the water. Why? Chlorinating old water can produce 'chemical smoke' from the reaction of the chlorine with the dirt. This 'smoke' (volatile chlorinated organics AKA disinfection by-products or DBPs) can (a) damage the cover and (b) remain in the pool until the pool is uncovered.

4. Continue chlorinating till the water is crystal clear and no slime or goo is felt or seen anywhere.

5. At that point, you're ready to 'prep' the pool water for 'warm storage'. But you've got choices to make. Chlorine will NOT reliably keep your pool for 3 weeks when the water temp is 75 degrees or higher. There are 4 available steps you can take, singly or all together: use polyquat, raise borates to 60 ppm, lower phosphates to below 200 ppb, add copper algaecides. The first 3 have no negative side effects; copper can stain pools and turn swimmer's hair green.