Katie speaks Scottish, and so uses the original Scottish/English meaning of billion:
https://www.theguardian.com/notesandqueries/query/0,5753,-61424,00.html
(A little outdated, perhaps, but nobody said Bellanoch was a modern kinda place)
Now something I always wondered makes sense, the fact that Billion is local usage. Metric and scientific use tera for 10^12 and Giga, for 10^9, as in microwave giga-hertz or computer giga-byte.
BTW, a UK gallon is bigger than a US gallon, so 3 tera gallons US is 11.35 tera liters. 3 tera gallons UK is 13.64 tera liters.
If you want, you can start to refer to it as a 12 tera liter, or 12,000 giga liter mana pool starting at act X
Fun fact: 3 tera-gallons is enough to supply an 8 million population city such as London or New York City with 100 gallons of water per person for 10 years. (((edit: 100 gallons per person per day)))
In the US, large quantities of water are refereed to as acre-feet by farmers, enough water to cover 1 acre 1 foot deep, or 326,700 gallons US, 272,250 gallons UK, which makes the entire mana pool 10 million acre feet, or 1.25 million hectare meters if you prefer metric.