Captain Sam Warburton, halfback Conor Murray and lock Alun Wyn Jones are all currently injured.
They were named last month in the 41-man squad for the tour of New Zealand tour, beginning next month, and remain on course to be fully fit.
Warburton is sidelined with a knee injury but he could play for Cardiff ahead of the Lions' departure for Auckland on May 29.
Ireland star Murray, meanwhile, has not played since mid-March due to a shoulder problem, and Warburton's Wales teammate Jones also has a shoulder injury.
"He [Warburton] is good," Lions head coach Gatland told Sky Sports News HQ.
"I spoke to Sam yesterday, and he feels that he could play now, and so he has made some really good progress and is really happy with how he has responded to treatment over the last few weeks.
"I expect him to play before he goes to New Zealand.
"[Jones] is making good progress as well. [Lions assistant coach] Steve Borthwick went down and saw him last week, and he's confident that he is going to be okay as well.
"Conor Murray, the prognosis with him looks pretty positive in terms of hopefully getting back and getting a game for Munster as well, and that will be good for him over the next few weeks if he can get a game."