Recruit Dylan Roberton has also been rewarded for a strong first season with the Saints, signing a two-year contract extension.
AFL.com.au reported in June that Webster had re-signed and Newnes and Ross were on the verge of penning new deals, with all three now committed until the end of 2015.
Regular members of the senior team this year, their signatures are a significant boost to the Saints as they undergo a rebuild under coach Scott Watters.
St Kilda has now retained its top four selections from the 2011 NAB AFL Draft, with injured forward Daniel Markworth contracted until the end of 2014.
Roberton has slotted in as a key member of the Saints' back six, playing every match this season and reaching the 50-game milestone last week against Richmond.
He is ranked No.7 in the AFL for overall rebound 50s (53) and has averaged a career-high 17.1 disposals, helping the Saints set up from the back half.
Watters recently said the Saints had the AFL's "worst retention rate from a recruitment point of view" but the pieces are starting to fall into place.
Key youngsters Jack Steven, Ben McEvoy and Arryn Siposs are all contracted until the end of 2015, while David Armitage re-signed earlier this season until the end of 2016.
Ross has shown promise as an inside midfielder this season, averaging 15 possessions in 10 games, including a personal best 22 against the Western Bulldogs in round nine.
The 20-year-old was recruited with pick No.25 in the 2011 Draft, playing one game in his debut season.
Webster has played the past seven games after making his debut in round seven, fitting into the team as a tough half-back with a damaging left foot kick.
Recruited with pick No.42 in 2011, the 19-year-old has impressed after shin splints restricted him to three VFL games in his first season.
Newnes, 20, has missed just two games this season, after he suffered concussion against Greater Western Sydney in round three.
Recruited with pick No.37 in 2011, he also played seven games in his debut season, slotting in across half-back.
Roberton, who played 37 games for Fremantle, asked the Dockers to trade him to a Melbourne club last year so he could be close to his family following the birth of his first son, Boston.
Recruited with pick No.49 in the 2009 NAB AFL Draft, he was never able to cement himself in the Fremantle backline, playing 10 games last season – the fewest of his three-year career.
At 191cm, he shapes as a versatile option for the Saints, who could also use him in lockdown roles as his career progresses and he adds strength to his frame.
Nathan Schmook is a reporter for AFL Media. Follow him on Twitter: @AFL_Nathan