How can you correct the pH of a pool with a reading of 3?
My recommendation: empty the pool and start over.
If your pool has pH 3, you might as well be swimming in Diet Pepsi.
Assume that your pool is 3.7 m long, 1.8 m wide, and 1.5 m deep
(about 12 ft × 7 ft × 5 ft).
Its volume is 10 000 L (2600 US gal).
I won't do the calculation here, but it would take about 1.1 kg (2.3 lb) of soda ash (sodium carbonate) to raise the pH from 3 to 7.
In contrast, it takes only about 100 g (2 oz) of soda ash to raise the pH from 7 to the recommended level of 7.5.