Twins Jamie and Craig Overton have agreed new contracts with Somerset, keeping them at the county until 2018.
The brothers, 21, were both called into England's one-day squad against New Zealand last month.
Jamie is also set to train with the England squad ahead of the first Ashes Test in Cardiff on Wednesday.
Seamer Lewis Gregory has also signed a new three-year deal and wicketkeeper Michael Bates will stay with the county until the end of the season.
Gregory, 23, a former England under-19 captain, was also called by the national team in May for a one-dayer against Ireland.
The right-armer was not selected for the abandoned match in Malahide, but has been a mainstay of Somerset's side in all formats this season.
"I'm delighted," Somerset director of cricket Matt Maynard told BBC Somerset. "Those boys have been very impressive.
"It was a very easy decision from my point of view to extend those three. I'm very happy they are here for three years and I think they will form the core of the team over the coming years."
Bates, released by Hampshire at the end of last season, had been on a one-month contract at Taunton.
The 24-year-old made his debut for the county against Nottinghamshire in June and has been rewarded for his early performances with an extended deal.
Maynard now says Bates must start scoring with the bat having impressed as wicketkeeper this season.
"You don't really notice him," he said. "He has added a little bit to our side behind the stumps, he knows he needs to score runs and hopefully he will now have the opportunity to show us what he can do with the bat."