Dillon has signed a two-year deal, while Sironen has put pen to paper on a one-year contract.
Dillon joins from the Sharks after making four appearances for the club this season, with his debut coming in Round 6.
The 24-year-old also represented the 2018 NSW Residents this season.
Sironen makes the move from the Wests Tigers, after playing two NRL games with the club last season.
He also represented the 2018 NSW Residents, and was named in the NSW Emerging Blues squad this season.
“Both Kurt and Bayley played NSW Residents this year, which shows they are consistent performers at that level below first grade,” South Sydney coach Anthony Seibold told rabbitohs.com.au.
“They have both made their NRL debuts and have shown great potential, and that is why we have given them an opportunity to join our Club and push for spots in our top 17.
“We know they can improve as footballers in our environment and we’re looking forward to working with them during the pre-season ahead of 2019.”
Tatola played 23 NRL games this season, while Nicholls featured in 12.
Gagan has played eight NRL matches in his career with Cronulla and Newcastle, and has been a mainstay in the Intrust Super Premiership.
“These three are key signings to help our depth in both the forwards and the backs, and to keep the pressure on spots in our top 17,” Seibold added.
“Junior has done a remarkable job playing in 24 straight games in first year of NRL Football, stretching back to the Charity Shield in February.
“‘Nico’ (Mark Nicholls) adds experience and depth to our front row rotation and has proven this year that he can play consistent NRL-level football, and Jacob has shown a great attitude towards his training and his preparation, and he will add to that competition for positions in our backline.”