The couple had only produced one hatchling before the three popped up.

Source: https://nypost.com/2023/03/20/90-year-old-tortoise-mr-pickles-welcomes-three-newborns-at-houston-zoo/

“We have 90 tortoises with transmitters, both head-starts (hatchling tortoises reared in captivity to maximize growth rates) and adults.

Source: https://yellowhammernews.com/david-rainer-shelton-nix-a-repeat-usfws-award-winner/

While adults can't survive subzero temperatures, hatchling use supercooling to prevent their bodies from freezing while overwintering in their nests.

Source: https://www.cbc.ca/news/canada/london/wildlife-surviving-winter-1.6735263?cmp=rss