Temba Bavuma, Ryan Rickelton, and Wiaan Mulder are set to feature as they look to build match fitness and regain rhythm before the international season.
Lions first-class captain Dominic Hendricks welcomed their inclusion, saying the trio would significantly strengthen the side.
"It is good to have those three guys back, they bolster our squad massively," said Hendricks.
The experienced players were key members of South Africa's successful Test team last year, and their return will be particularly valuable with Kagiso Rabada unavailable through injury.
The Lions are therefore expected to field a relatively inexperienced bowling unit, with Rafiq Patel and Dayalan Boyce among those likely to receive opportunities.
The Lions have claimed or shared the four-day title in each of the past three seasons, but their squad depth will face another stern examination.
The expanded competition now features 10 matches for each of the eight Division 1 teams, increasing the demands across the domestic calendar.
Several Lions players, including Codi Yusuf, Nqaba Peter, Delano Potgieter and Lutho Sipamla, could also be required by the Proteas or SA A during the season.
Youngsters Jason Rowles and JJ Basson are expected to gain valuable exposure, while 21-year-old Richard Seletswane enters the campaign after striking an unbeaten 150 for SA Emerging against Bangladesh Emerging.
Ryan Rickelton and Temba Bavuma shone with the bat in the opening round of the domestic first-class competition, but there was an injury concern for Proteas fast bowler Lungi Ngidi who hobbled off injured.
The Proteas will be without pace bowler Ottneil Baartman for their ODI series against Australia after he injured his hamstring playing first-class cricket for the Dolphins.
The Proteas wrapped up their ODI series against Namibia with a commanding nine-wicket victory in Windhoek on Sunday, completing a 3-0 clean sweep.
Connor Esterhuizen struck his maiden ODI century as the Proteas beat Namibia by 157 runs in the second match in Windhoek, with the visitors wrapping up a series victory with a game to spare.
