Prop Slipper and second row Simmons both came off the bench in all three internationals in Brisbane, Melbourne and Sydney, while openside flanker Gill did the same in the first and second Tests before missing out altogether following George Smith's recall for the final encounter.
All three have re-signed with the Reds until the end of 2015 and, with former Queensland boss Ewen McKenzie replacing Robbie Deans in the Wallabies hot seat, could have even bigger roles to play in next month's Rugby Championship and beyond.
First up is a shot at a second Super Rugby title in three seasons, though, with the Reds traveling to the Crusaders for the first qualifier this Saturday.
"What we've achieved at the Reds over the past few years has been special. It's a privilege to train with and play with the group of guys we have here and the setup we have in place will also ensure we can maintain the same level of success in coming years. That's exciting," said Slipper, who made his 50th appearance for the Reds in last week's win over the Waratahs.
"There is a very good culture and I'm really looking forward to what is in store over the next two years. Not only do I love playing for Queensland, but also with my teammates here and that was a major motivation to re-sign.
"The World Cup coming up is also a major influence and you want to be a part of that. I feel playing for the Reds gives me the best opportunity to do that."
Simmons is the most experienced of the three players having won 64 Super Rugby caps for the Brisbane-based side and, like Slipper, he hasn't missed a competitive fixture for his franchise over the last three seasons.
"Staying in Queensland is what I always wanted to do and I've got aspirations here and also for the Wallabies," added Simmons.
"It all comes down to the culture. There are a lot of guys who take pay cuts to stay in Queensland and that's a credit to this squad. That's the reason I want to stay: it feels like a family to me, and I'm confident we've got the systems in place to be successful for a long time."
Those sentiments were echoed by 21-year-old Gill, who is expected to provide David Pocock and Michael Hooper with stiff competition for the Wallabies No7 shirt in the lead up to the World Cup.
Gill has been in sensational form in the Super XV and was unfortunate not to start ahead of Hooper or Smith against Britain and Ireland's elite after heading into the series as arguably the form flanker in the southern hemisphere.
"I've very excited and relieved that I've got the chance to stay in Queensland. I didn't negotiate anywhere else because this is the place I want to be," said Gill.
"We've had three very successful seasons under Ewen and now Richard (Graham) will be our head coach for the next two years. We've got a taste of that this year and I'm excited about why lies ahead for this team.
"We've also got a great bunch of blokes here and I get along with them both on and off the field. It's a real honour to play under the Queensland badge and alongside a lot of great players at the club."