Tt will take quite a while, 1TB aught to be enough for everyone.
Kidding aside, you can assume that in 10 years storage will be about 1000 times cheaper then it is now, so 3PB of storage would be around the cost of 3TB drive today, around 100$.
Data storage today certainly isn't 1000 times cheaper than it was a decade ago, I'm not sure it's even 100 times cheaper.
For some reason you seem to believe that storage prices will remain the same while we double capacity every 12 months, that isn't even close to reality.
(Even if you've misapplied Moore's Law to flash storage, that predicts doubling every 2 years not every year.)
I've looked at the prices between 1980 and 2010, seemed to be getting about 1000 times cheaper every 10 years.
And SSD pricing has gone down about 40% every year the last 4 years. Which would come to a price reduction of less, but some breakthrough is bound to be around the corner.
Kidding aside, you can assume that in 10 years storage will be about 1000 times cheaper then it is now, so 3PB of storage would be around the cost of 3TB drive today, around 100$.