Full adult size at 3 months is not typical for betta bred for commercial purposes. It is only achievable through power growing, 100% water changes or 100s+ gallons of water to dilute growth hormones and power feeding high protein/fat food multiple times a day, usually only done by people that show betta and want "quicker" results from their projects.
Most betta we see at the store that are "full adult size" tend to be older at around 6+ months of age.
kinda off topic but slow growth is not necessarily bad, a lot of older fish/betta keeper and myself believe betta fish used to have longer lifespans of 4-8 years back when heaters weren't readily available for the hobby, compared to most betta fish that have a lifespan of 2-4 years these days with heaters. It could be due to poor genetics from too much line breeding, but in my opinion it has a lot to do with the speeding up of their metabolism/growth rates with constant high temperatures.