The contract extensions mean the trio will also remain at the Queensland Reds in a boost for the Brisbane-based Super Rugby team.
The 21-year-old Gill, who has impressed in his 10 Test caps since his debut against New Zealand last year, bolsters an impressive array of Wallabies back-rowers which includes David Pocock, Scott Higginbotham and Michael Hooper.
"The rugby World Cup in 2015 is a long-term goal but that is a long way down the track and anything could happen," Gill said.
"I'm certainly excited to be playing in Australia and for Queensland and if I'm lucky enough to fall under the Wallaby category, I can't wait for it."
Slipper has notched up 37 caps since his 2010 debut and the 24-year-old prop will be expected to take the pressure off seasoned front rowers Benn Robinson and Ben Alexander.
Lock Simmons, also 24, partners Australia captain James Horwill in the second row at the Wallabies and the Reds and will hope to add to his tally of 26 Test caps following the retirement of veteran Nathan Sharpe.